Patrick Just Math Tutorials: "Polynomials: Adding, Subtracting, Multiplying, and Simplifying" video
NOTE: All Khan Academy content is available for free at www.khanacademy.org.
You might have heard you can only combine like terms. Why is this? What if a classmate didn't believe you? For example what conclusions would you be able to draw if 2x + 1 = 3x for all x?