You may want to review factoring polynomials first.

Common denominators of rational functions are the same as for
fractions. You need to find a polynomial which both denominators
go into evenly. The smaller the polynomial is, the better. For
example, suppose you had to find a common denominator for
1/(*x* + 1) and −1/*x*. You would want to
use *x*(*x* + 1), because *x* and *x* + 1 both
go into that evenly. Of course, this multiplies out to
give *x*^{2} + *x*. Another common denominator
would be 2*x*^{2} + 2*x*, but that wouldn't be
as good. You should always try to use the lowest common
denominator (LCD, for short).