If 2-i is a zero, then (x-(2-i)) or (x - 2 + i) is a factor of G(x). We need to find the other factor and for this we can use Synthetic Division:
2-i | 1 -1+i -8+i 12-6i
----- 2-i 2-i -12+6i
----------------------
1 1 -6 0
Te remainder is zero so 2-i is indeed a factor. And the rest of that row of numbers, 1 1 -6, tells us the other factor: . So now
The second factor is a trinomial that is easily factored giving:
G(x) is now fully factored.