I guess you don't know newton's method to find zeros. Therefor you have to find a zero by guessing
f(z) :=z^3 - 3z^2 + 6z -4
very soon you find that f(1) = 1-3+6-4 = 0
Then use polynomial long division to get
[z^3-3z^2+6z-4] / [z-1] = z^2 - 2z + 4
I guess you do know how to find the complex zeros of this polynom.