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)