How do you Factor this
9x - 81
divide each side by the largest number that they are both divisible, in this case it is 9. so divide 9x by 9 and -81 by 9. As a result you get x-9, which is your answer.
9 ( x - 9) . Nine is the greatest common factor for that expression so you would use that to factor into the terms which are 9x and -81.