discrete mathematics – prove that the whole is countable

Show that the whole {x ∈ Z | x ≥ 3} is countable.

I am really stuck on this problem, I have a review very soon and I am trying to solve it. Until now, I know that I have to show that it's on and one-on-one.
Would I start by saying that
f (n) = 3 + n
and then show that it's on and one-to-one?