Answer:
r = -1
Explanation:
Slope between two points is determined by (y2-y1)/(x2-x1)
In this case, we would get (-8-0)/(8-r), making:
-8/(8-r) = -8/9
8-r = 9 because you want the denominator to equal 9, and therefore when you input r as -1, we get the denominator to have the value of 9.