The order is the cardinal of the set in which you have the group structure. Have you talked about the 2nd isomorphism theorem? If yes, think how you can apply it to this problem.
Let G be an Abelian group. Let H and K be subgroups of G with orders 36 and 45, respectively. Which of the following is a possible order of HK?
This doesn't seem difficult, but I've never taken Abstract Algebra before... So I don't even know what it means to find the order of a subgroup.