f(-3)=36-(-3)-3=36 and f(-2)=16-(-2)-3=15.
So we have two points (-3,36) and (-2,15). The average rate of change is the difference of the y coords divided by the difference of the x coords, to give us the slope=(15-36)/(-2-(-3))=-21/1=-21. This slope is the average rate of change between x=-3 and x=-2.