It says solve by factoring--- x^2 + 2x = 15 I know the answer is x=3 or -5, but I have no idea how to get there by factoring. Any help would be very much appreciated since on the test we will have to show our work.
$\displaystyle x^2 + 2x - 15 = 0$
List the pairs of factors of -15:
1, -15
3, -5
5, -3
15, -1
Which of these add to 2? 5 + (-3) = 2. So
$\displaystyle x^2 + 2x - 15 = 0$
$\displaystyle x^2 + 5x - 3x - 15 = 0$
$\displaystyle (x^2 + 5x) + (-3x - 15) = 0$
$\displaystyle x(x + 5) - 3(x + 5) = 0$
$\displaystyle (x - 3)(x + 5) = 0$
-Dan