Seeing as we have a Diophantine Equation with mn, m, and n, it's probably a good idea to use Simon's Favorite Factoring Trick. Move all the terms to one side:
\(mn-2m-4n=0\)
Factor out an m from the first two terms:
\(m(n-2)-4n=0\)
I will leave it to you to figure out what to do next.
Hint: Try to find a way to get another (n-2). You already have an -4n.
