To find the other endpoint with a given endpoint and midpoint, you subtract the midpoint values from the endpoint, then with whatever the answer is, you subtract that from the midpoint.
ANSWER: y=x-8
EXPLANATION:
9 x 72 = 9 x ? + 9 x 2
72 = 8 x 9 = 4 x 2 x 9
9 x 72 = 9 x 4 + 9 x 2