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
