Determine The Equation of a Line

Lesson Objective

This lesson shows you how to determine the equation of a line using the given information such as the slope, coordinates of a point and more.

Study Tips

Tip #1

You need to have some knowledge on Slope-Intercept Form of a line. You can learn about it by watching the math video in this lesson.

Tip #2

Usually, to determine the equation of a line, we should begin with the Slope-Intercept Form of a line (as shown in the picture).

Next, we just use the information given to find the slope (m) and the y-intercept (b).

However, bare in mind that the information given may not directly help us to find the slope or y-intercept. If this is the case, we need think of way of using the information to find them.

Now, watch the following math video to learn more.

Math Video

Click play to watch video

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Math Video Transcript

Determine the Equation of a Line Transcript 

00:00:01.120
In this lesson, we will learn how to determine the equation of a line, using the available information.

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Let's look at the first example. Determine the equation of the line, that passes through the points (2, 5), and with the slope of 2. 

00:00:19.000
To begin, we should know that the equation of a line can be written in the form of y = mx + b, where m is the slope, and b is the y-intercept.

00:00:30.110
From this, to determine the equation of the line, we just need to find the values of m and b.

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Now, we can see that, the slope m, is already given as 2. 

00:00:43.050
Therefore, we can just substitute m with 2, and the equation now becomes y = 2x + b. 

00:00:51.070
Next, we need to find the y-intercept, b.

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Since the value of b is not given, we need to find it. 

00:00:59.220
To do so, we know that the line passes through the point 2, 5. 

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Therefore, we can use this point by substituting x with 2, and substituting y with 5. 

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With this, notice that we can now solve for b. 

00:01:18.010
To solve for b, multiply 2 with 2. This gives 4. 

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Next, add negative 4 to both sides of the equation. This gives 5 - 4 =  b.

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5 minus 4 gives 1. Hence, we found the y-intercept, b as 1.

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With this, we can write  b as 1.

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Finally, since we found both m and b,  the equation of the line is y = 2x + 1

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Now, next example.

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Determine, the equation of the line, that passes through the points (1,4) and (2,1).

00:02:04.190
Again, we should know that the equation of a line can be written in the form of y =  mx + b.

00:02:12.060
We can see that, the slope and y-intercept are not given. Instead, we only have the coordinates of 2 points. 

00:02:20.210
With some thinking, we can use these 2 points to find the slope 'm' by applying the slope formula, (y2-y1)/(x2-x1).

00:02:32.140
To use the slope-formula, we can assign this point as point 1, with the x-coordinate as x1, and y-coordinate as y1. 

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Similarly, we assign the next point as point 2, with x-coordinate as x2, and y-coordinate as y2.

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Now, we can find 'm' by just substituting, y2 with 1, y1 with 4, x2 with 2, and x1 with 1. 

00:03:04.100
Alright, we can remove these brackets, as they do nothing.

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Let's calculate 'm'. Negative multiply by bracket 4 gives negative 4. negative multiply by bracket 1 gives negative 1.

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1 minus by 4 give negative 3. 2 minus by 1 gives positive 1.

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Negative 3 divides by positive 1 gives negative 3. 

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So, we found the slope 'm' as negative 3. Now, we can write m as negative 3.

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Next, we need to find the y-intercept, b. 

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Now, similar to the previous question, we can find b by taking a point on the line, and substitute its x-coordinate and y-coordinate into the equation.

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Let's take this point 1,4.

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Substituting x with 1 and y with 4.

00:04:07.220
Now, we can solve for 'b'. Multiplying negative 3 with 1 gives negative 3.

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Next, add  positive 3 to both sides of the equation. This gives 4 + 3 = b.  

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4 plus 3 gives 7. Hence, we find the y-intercept, b is 7.

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With this, we can now write b as 7.  

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So finally, with both slope and y-intercept found, we have the equation of the line as, y =  negative 3x + 7

00:04:44.060
That is all for this lesson. Try out the practice question to further your understanding.

Practice Questions & More

Multiple Choice Questions (MCQ)

Now, let's try some MCQ questions to understand this lesson better.

You can start by going through the series of questions on determining an equation of a line or pick your choice of question below.

Question 1 on determining the equation of a line using the coordinates of two points.

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