Given the perimeter of a triangle is 18ft. Its side lengths are 5ft, 7ft and x ft. Find x.

The picture below shows the triangle:

We start with the formula for the perimeter of a triangle, P, as shown below.

Since the values of the perimeter and the side lengths are given, we can substitute these values into the formula as shown:

To find x, we need to remove 12. To do so, we add -12 to both sides of the equation:

Now, we have x = 6. Remember that, it is important to include the unit. Hence:

Therefore, the answer is B.

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