Purple Math
Purplemath: Descartes' Rule of Signs
Purplemath provides examples that helps explain Descartes' rule of signs, which will help you find the zeros of a polynomial.
Wolfram Research
Wolfram Math World: Descartes' Sign Rule
This page gives a good overview of Descartes' Sign Rule, a "method of determining the maximum number of positive and negative real roots of a polynomial."