Answer: 12
Work Shown
The interval notation [3, 5] means 3 ≤ x ≤ 5
Plug x = 3 and x = 5 into the function to find f(3) = 8 and f(5) = 32.
Then we use the average rate of change formula.

Alternatively, you can use the slope formula to find the slope of the straight line through the endpoints (3,8) and (5,32).