Answer:
r=8
Step-by-step explanation:
The formula for slope is
m = (y2-y1)/ (x2-x1)
Substitute in what we know.
8/3 = (-7-1)/(5-r)
Simplify
8/3 = -8/(5-r)
We can use cross products to help us solve.
8 * (5-r) = 3 * (-8)
divide each side by 8
8/8 * (5-r) = 3 * (-8)/8
5-r = -3
Subtract 5 from each side
5-r-5 = -3-5
-r = -8
Multiply by -1
r = 8
