In proof questions, what can any even number be written as?

An even number is a number divisible by 2.

Therefore, an even number could be written like this:

x is even if x = 2n, n∈ℕ.

This basically means that x is equal to twice some number n (where n is a natural number). This makes x an even number because 2 times any natural number is an even number (so 2n is always an even number); therefore x = 2n is even.

If that is how you define x, you can then use it in your proof as any even number.