You may want to review factoring one-variable polynomials first.

If a polynomial looks like *x*^{2} −
5*xy* + 4*y*^{2}, you can deal with it the same
as if the *y*s weren't there. Just look for two numbers which
multiply to 4 and add to −5. These are −1 and
−4. So the answer is (*x* − *y*)(*x*
− 4*y*). (The first factor would be *x* −
1*y*, but of course 1*y* is the same thing
as *y*.)