Answer: x+2
Explanation:
It is not factorable through grouping method. You can test for factors if you divide the factor and it goes in evenly (no remainder) then, it is a factor.
I will use synthetic division to test:
X-8:
8 | 1 -14 56 -64
| 8 -48 64
1 -6 8 0 =>no remainder so yes this is a factor
X+2:
-2 | 1 -14 56 -64
| -2 32 -176
1 -16 88 -240 => remainder so NOT factor
X-2:
2 | 1 -14 56 -64
| 2 -24 64
1 -12 32 0 =>no remainder so yes this is a factor
X-4:
4 | 1 -14 56 -64
| 4 -40 64
1 -10 16 0 =>no remainder so yes this is a factor