When you factor it, -9 + 1 = -8.

Although it should be (x+1)(x-9).

In a form like that, when you multiply it out, you get x^2 + x -9x -9. Which is x^2 -8x + 9.

When factoring, it does the opposite.

Basically think of it as (x+a)(x+b) = x^2 + (a+b)x + (a*b). In this case, 1 and -9 are a and b, which added is -8.