What is the nth even number?

I ume you're referring to the positive even numbers. So 2,4,6,8,... You might notice that these can be written as 1*(2), 2*(2), 3*(2), 4*(2),... It's not too hard to see then that the nth positive even number is of the form 2*n. You'll need to tweak this slightly if you are including 0.