iff means you have to do it both ways:
x even => x^3 even
x=2k
x^3=2^3k^3
x^3=2(4k^3)
x^3=2m
=> even
x^3 even => x even
x^3=2k
...
Hey you do the rest!
-O
Then you know nothing about mathematics!
Mathematics is only about proofs.
Numbers have almost nothing to do with mathematics.
In fact, people who cannot do proofs cannot do mathematics.
I fear that your passion is simply and tragically misplaced.
So you must learn to do logic and proofs.