Answer:
Step-by-step explanation:
1) x² + 2x - 15 = 0
<=> x² - 3x + 5x - 15 =0
<=> (x+5)(x - 3) = 0
<=> x = -5 or x = 3
2) x² + 5x + 6 =0
<=> x² + 2x + 3x + 6 = 0
<=> (x+3)(x+2) = 0
=> x = -3 or x = -2
3) x² - 6x - 40 = 0
<=> x² - 10x + 4x -40 = 0
<=> (x+4)(x-10) = 0
<=> x = -4 or x =10
4) x² - 8x + 12 =0
<=> x² -2x -6x + 12 = 0
<=> (x-2)(x-6) = 0
=> x = 2 or x = 6