Adding and subtracting polynomials with two variables

The  exercise appears under the Algebra I Math Mission. This exercise practices adding and subtracting polynomial expressions with two variables.

Types of problems
There is one type of problem in this exercise:
 * 1) Express ___ - ___ or ___ + ___ as a trinomial.: This problem explains that __ and __ are trinomial expressions, and user is asked to express ___ - ___ or ___ + ___ as a trinomial.

Strategies
Basic knowledge of addition is required for this exercise.
 * 1) When user is asked to find  ___ - ___ or ___ + ___, they need to substitute in the polynomial expressions that we are given for __ and __.