Put this into this form:
It is like factorising a number.
You can divide m^2 by 2 without getting a remainder, but a quotient equal to n^2.
I hope it helps.
If m^2 is twice n^2 that means m^2 is even. (Which is the same as saying 2 is a factor of m^2.) But if m^2 is even then m must also be even.
As it turns out, we know something stronger: that 4 is a factor of m.
Are you looking at a proof that the square root of 2 is irrational, by chance?