Let f be an isomorphism between S and T
let s,t\in T . Given that S is commutative, we want to show that st=ts.
Since f is isomorphism, f is onto. So there are a,b such that f(a)=s and f(b)=t
=f(a\bullet b) by definition of homomorphism
=f(b\bullet a) since S is commutative
=f(b)f(a) again by homomorphism
For associativity you may try by yourself first