(x - 9)^2
Whenever dealing with a problem like this the first thing you should consider is a square in the last term. Factoring trinomials is always a guess and check procedure, but there are multiple tips to enhance and quicken your guessing. When the last term is a positive square, find the square root of it and double it. Chances are, that result will be the middle term. It is only a matter of addition of subtraction now. If the middle term is negative then it is a subtraction, and if the middle term is positive then vice versa.
81 is a positive square and the square root is 9. By doubling it you get 18. The middle term is negative, so the binomial squared is (x - 9). Hope this helps!
