When factoring a binomial such as
4x^2 - 81, look to see if either of the terms are perfect squares. If they are, factoring is very simple.
Take the square root of each term to get
2x and 9
(notice we ignore the - on the 81)
The factored form is just two factors with each term, one with a + in between and one with a - in between
(2x + 9)(2x - 9)
If the terms of a binomial are not both perfect squares, then look for a greatest common factor to factor out.