Suppose I have 1 book A and 1 book B. How many combinations are there if I select 1 of each type?
1 * 1 = 1

Suppose I have 2 Book A and 2 book B. How many combos, once again selecting 1 of each?
2 * 2 = 4

Thats how you do these sorts of problems.
What are they teaching you guys in school nowadays, anyways?