5x³ + 8x² - 7x - 6
1)
x + 2 is a factor because when using synthetic division, the remainder is zero.
-2 | 5 8 -7 -6
| ↓ -10 4 6
5 -2 -3 0 ← remainder is zero
2)
x + 2 is not a factor because when using synthetic division, the remainder is not zero.
-1 | 5 8 -7 -6
| ↓ -5 -3 10
5 3 -10 4 ← remainder is not zero