Khan Academy
Khan Academy: Add & Subtract Polynomials: Two Variables (Intro)
Example: Subtract or add polynomials. Students receive immediate feedback and have the opportunity to try questions repeatedly, watch a video or receive hints.
CK-12 Foundation
Ck 12: Algebra Ii: Addition and Subtraction of Polynomials
[Free Registration/Login may be required to access all resource tools.] Here you'll learn how to add and subtract polynomials, as well as learn about the different parts of a polynomial. Practice problems are also included.
University of Saskatchewan (Canada)
University of Saskatchewan: Polynomials and Roots
This site, which is given by the University of Saskatchewan, provides a general overview of polynomials and roots. Examples and exercises are included.