If R is an equivalence relation on S, and a is an element of S, define the equivalence relation of a by [a]={x in S: aRx} for the following relation.

Let H be a subgroup of G, define R on G by saying aRb iff a-b is an element of H.

Find the equivalence class of a.

I have already proved that the above relation is an equivalence relation. I just don't know how to find the equivalence class of a. It can't be that hard, but I'm not sure where to start.

Thanks.