Combinational Methods help

Hey everyone,

I just started my statistics course and I'm having trouble with one of the questions. I was hoping that somebody could give me some insight into completing one of the first ones, because every question after it references back to it.

Here's the question:

"An operation consists of two steps, of which the first can be made in ways. If the first step is made in the th way, the step step can be made in the ways.

A) Use a tree diagram to find a formula for the total number of ways in which the total operation can be made.

B) A student can study 0, 1, 2, or 3 hours for a history test on any given day. Use the formula obtained in part A to verify that there are 13 ways in which the student can study at most 4 hours for the test on two consecutive days."

The formula in the back of the book is

(that n on top should be but I can't seem to get it to show up that way, hope it still makes sense)

which I sort of understand, but if somebody could give me a more thorough explanation I certainly would appreciate it.

For the tree diagram, I believe I drew it correct, but I don't know how to apply the formula to this question. I'm just stuck overall.

Any bits of help are greatly appreciated, thanks!