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 2x + 3 by 2x − 3. The product is then the square of the first term minus the square of the second term. In this case, that's (2x)2 − 32 = 4x2 − 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.