Are you taking an "Abstract Algebra". If so you should know what "isomorphic" means- that there is a function from one group to the other, f: G->H, that
1) it is "one-to-one"- if f(x)= f(y) the x= y.
2) it is "onto"- for every y in H there exist x in G such that f(x)= y.
3) it "preserves the operation"- f(x+ y)= f(x)+f(y).
G is defined as multiples of 2 and H is defined as multiples of 3. What about f(2n)= 3n?