As for part (a), you are almost right. There is just one number, which is even and a multiple of 3, namely 0. Certainly 0 is even, and since 3 times 0 is 0 itself, 0 is a multiple of 3.

So A = {0}.

EDIT: As for the set B, CaptainBlack is correct over what I wrote, which was:

As for B, I take the description of B to mean that x is an element of B, if x is odd, and x is a multiple of 5. Only two members of S have this property, namely -5 and 5 (because 1 times 5 is 5, and -1 times 5 is -5).

So B = {-5,5}.