I've been stuck on this one problem for a good hour and I just don't understand how to solve it. The directions are as follows: For the following exercises, find the average rate of change of each function on the interval specified for real numbers b or h in simplest form.

11) a(t) = 1/t+4 on [9, 9+h]

I know the answer is -1/13(13+h), though I don't understand why that is. I would appreciate a step-by-step explanation.