I have a question here that says:

Let A = {1,2,3,4,5,6} and let S =P(A), the power set of A.

a) For a,b belong to S, define a ~ b if a and b have the same number of elements. Prove that ~ defines an equivalence relation on S.

b) How many equivalence classes are there? List one element from each equivalence class.

Could someone steer me in the right direction? Thank you.