Can't quite tell what you mean by n3.
1) If that's n3, then:
If n3 is even, it can be expressed as n * n * n = 2 * t (t is a natural number).
You can see that two is a factor of n3, and so it has to be a factor of n, because there is no ther number that could contribute to it. So n is even too.
2) If you mean 3 * n, then:
3 * n = 2 * t, where t is a natural number.
Again, the only number to contribute factor 2 for the right side of equation is n.
So n has to be even too.