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