How do you find the average rate of change through an interval

In order to find average rate of change over an interval, do the y2-y1/x2-x1. Solve y for x=9 and x = 4

y2 = - 3 (9-4) ^2 + 3 = - 72

y1 = - 3 (4-4) ^2 + 3 = 3

Now, x2 matches with y2 and and x1 matches with y1 obviously ...

-72 - 3/9 - 4 = - 15