Final answer:
The average rate of change from x=0 to x=4 is 15 units/year.
Step-by-step explanation:
The average rate of change from x=0 to x=4 can be found by calculating the difference in enrollments between these two points and dividing it by the difference in time. In this case, the enrollment at x=0 is 15 and at x=4 is 75, so the change in enrollment is 75 - 15 = 60. The change in time is 4 - 0 = 4 years. Therefore, the average rate of change is 60/4 = 15 units/year.