There is nothing to "do".
It is x^2 -x -12 is (x+3)(x-4) when factored
x = ? 42 = x squared minus x
(x - 12)(x + 12)
13X squared + 7X + 12
x2-10x-24 = (x+2)(x-12) when factored
(x + 12)(x - 3)
(x - 5)(x + 12)
5 or 12
Actually, x2 - 12 cannot be simplified.
x2 -x -12 = 0 (x+3)(x-4) = 0 Therefore, x = -3 & 4
(4+x) (2x-3)
By factoring I get x-3 divided by x+3