Fibonacci sequence induction proof

Hi!

I have a assignment involving the fibonacci sequence, and I wonder if anyone could give me a hand with it:

The fibonacci sequence is given as:

$\displaystyle f_{n} = f_{n-1} + f_{n-2}$

Using induction, prove that $\displaystyle f_{3n}$ is a even number for all $\displaystyle n \in \mathbb{N}$

I have been struggeling with this for some days now. One of my questions is how do I express the fact that the answer is a even number mathematically? Do I even need to?

Any help would be much appreciated!

Martin