You are taking a multiple choice test that has 4 questions. Each question has 5 answer choices, with one correct answer per question. If you select one of these choices for each question and leave nothing blank, in how many ways can you answer the questions

To expand it, you basically take two of everything in the parentheses and add addition or subtraction signs. Once you're done it will look like this:
x^2-y^2+x^2-y^2+2xy+2xy
Then you combine the like terms. In this expression, the like terms are x^2, -y^2, and 2xy. The final expression will look like this: 2x^2-2y^2+4xy