How to create a one to one correspondence between two sets? I am stuck with,

Give a one to one correspondence between Z+ and positive even
  integers.

Now, I don't have an idea how to show that there is a one to one correspondence between the two. I would be thankful for some hints.

Consider the f(x)=2x. Now if f(a)=f(b) then 2a=2b then a=b ( because $2\neq0$) this proves the function is injective, to prove that is surjective a positive even integer is of the form 2k with k a positive integer, then f(k)=2k.
