I have a question that I don't know the answer to.

Let F be a field. I showed that SL2(F) defined to be the matrix

(a b

c d) where a,b,c,d are in F and ad-bc=1 is a group.

I already proved that.

However, the question goes on to ask,

If F has q elements, how many elements are in the group SL2(F)?

I don't know the answer.