For the future use of metaname90 (and anyone else): the only reason you cannot just put x= 4 into $\displaystyle \frac{x- 4}{x^2+ 6x- 40}$ is because you get "$\displaystyle \frac{0}{0}$". And saying that x= 4 makes $\displaystyle x^2+ 6x- 40= 0$ tells you that x- 4 is a factor! Once you know that it should be easy to find the number, a, so that $\displaystyle x^2+ 6x- 40= (x- a)(x- 4)$.