Multivariate polynomials have at least one term that has more than one variable as a factor. You operate on multivariate polynomials using the same techniques you've been using with polynomials that have single variable terms. The first set of videos concentrates exclusively on multivariate polynomials and the second set of videos generalizes our ideas across all our polynomials. This material is covered on pages 209 through 218 in the Engaging Algebra text.
5.3 Operations with Multivariate Polynomials
After this section you'll be able to operate on multivariate polynomials.
5.4 Continuing with Polynomial Multiplication
After working with these videos you'll be able to simplify all your polynomials using the same small set of ideas.