If this is just factoring, x^2 + x - 90 turns into this:
(x - 9)(x + 10)
If your trying to solve for x, we take the factored expression and set it equal to 0.
(x - 9)(x + 10) = 0
Set each binomial into two equations.
x - 9 = 0 and x + 10 = 0
Solve for x and we get two answers.
x = 9
x = -10
