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

Now

st=f(a)f(b)

=f(a\bullet b) by definition of homomorphism

=f(b\bullet a) since S is commutative

=f(b)f(a) again by homomorphism

=ts

For associativity you may try by yourself first