calculus
posted by confusedplease help .
Find the average rate of change
f(x)=x^38x+9 a. from 3 to 2
b. from 1 to 5
c. from 5 to 9
A. what is average rate of change of f(x)=x^38x+9 from 3 to 2?
B. what is the average rate of change of f(x)x^38x+9 from 1 to 5?
C. what is the average rate of change of f(x)=x^38x+9 from 5 to 9?
Please show work

How about if someone shows you how to do one and you do the rest?
The average of f'(x) over an interval a to b is the area under the f'(x) curve, divided by (b  a).
That equals f(b)  f(a), because the area under f'(x) dx is the integral, f(x)
A. f(2)  f(3) = 8 +16 +9
(27) 24 9 = ? 
a. is the answer 6

That is not what I get adding the numbers I gave you, but maybe I made a mistake.
I get +11