To prove m^2 is even if m is even, you do this:

m = 2n
m^2 = (2n)^2
m^2 = 4n^2
m^2 = 2(2n^2)
Since 2(2n^2) is 2 times a number, it's even, meaning m^2 is even.

How do you do the reverse? I want to prove m is even if m^2 is even.