-x^4+8x^3-16x factor out a -x
-x(x^3 - 8x^2 + 16)
there are several ways to solve this you can use long division or synthetic division. Do not forget the place holder in the x place (0x). this is the factors! I tried x+1, x-1, x+2, x-2, x-4, x+4, x-8, x+8. The factors of 16.