Answer:
(x + 4)(x - 2)
Explanation:
For factoring when there's a coefficient on the leading variable, I use the "slip n' slide" method (my teacher called it that, I had no part in naming it), but when that fails, you can use the quadratic formula.
First, you "slip" the leading coefficient to the end, and multiply.
5x² + 10x - 40
x² + 10x - 200
Now, you factor it normally.
x² + 10x - 200
= (x + 20)(x - 10)
Then, you divide the 2nd part in each parentheses with the coefficient you slipped in the beginning. This is super important! I always had trouble remembering this step, so hopefully you won't.
(x + 20/5)(x - 10/5)
= (x + 4)(x - 2)
I hope this helped!