If a and b are two non-empty sets such that axb = bxa, show that a=b?

To prove that A=B, you need to show that A is a subset of B and that B is a subset of A. There is no need to assume that A≠B. We won't be proving this by contradiction. 
First, let a1∈A. Then for any b1∈B, since A X B ⊆ B X A, there exists a2∈A and b2∈B such that a1=b2 and b1=a2. Since b2∈B and a1=b2, we have that a1∈B. Since a1 was arbitrarily chosen, we have that A ⊆B. 
Second, let b1∈B. Then for any a1∈A, since A X B ⊆ B X A, there exists a2∈A and b2∈B such that b1=a2 and a1=b2. Since a2∈A and b1=a2, we have that b1∈A. Since b1 was arbitrarily chosen, we have that B ⊆A. 

Thus we have shown that A=B.