Someone please help me with this math problem?

Can someone me and do the problem plz so i can understand it more better

Let f(x) = 3 − x . find the average rate of change of f(x) from x = a to x = a + h and simplify your answer so that no single factor of h is left in the denominator.

Cody was 165cm tall on the first day of school this year, which was 10% taller than he was on the first day of school last year.