I forget what its name is, but it is a theorem that states that in a polynomial function in the standard form there are possible, at most, as many positive solutions as there are changes in sign for every term. Likewise is true for f(-1) that will give you the most possible negative solutions.
-----------------------
^ I believe you're thinking of Descartes' Rule of Signs.
Also, Descartes was responsible for the concept of the Cartesian Plane.