Question

The points (-1,r) and (15,1)lie on a line with slope 1/2. Find the missing coordinate r.

Answer 1

Slope is the change in y over the change in x:

Slope = (y2 - y1) / (x2-x1)

Replace the letters in the equation with the given information:

1/2 = (1 - -1) / (15 -r)

Cross multiply:

1 * (15-r) = 2 * (1- -1)

Simplify:

15 -r = 4

Subtract 15 from each side:

-r = -11

Because r is negative, you need to multiply both sides by -1 to make r positive:

r = 11