Which expression is a factor of x^2 + 2x - 15?

To factor this you need values that add up to 2 and multiply to - 15

So you get 5,-3 then you put it into your equation for you b value

X^2+5x-3x-15

Then you can factor by grouping

X (x+5) - 3 (x+5)

So your factors are

(X+5) (x-3)