For any function f(x), the average rate of change on the interval [x1,x2] is given by [f(x2) - f(x1)]/(x2-x1)
For f(-1) = 2.5 and f(3) = 40, the average rate of change on the interval [-1,3] is given by:
step 1 - (40 - 2.5)/[3-(-1)] = 37.5/4 = 9.375
step 2 - (37.5)/4
step 3 - Solving the fraction we find that the average rate of change is 9.375 (or 75/8)