You may want to review multiplying polynomials first.

One special case of multiplying polynomials is when you need to
multiply two binomials which are the same except that one has a
plus sign in the middle while the other has a minus sign. For
example, you might need to multiply 2*x* + 3 by 2*x*
− 3. The product is then the square of the first term minus
the square of the second term. In this case, that's
(2*x*)^{2} − 3^{2} =
4*x*^{2} − 9.

Be sure the signs in the middle are different; otherwise this won't work.

Note that you can always use the regular method for multiplying polynomials; this trick just gets you the answer a bit faster.