If, say, x = 3 were a zero, then x - 3 would be a factor.
If, in this case, x = 1 + i is a zero, then what is a factor?
Since complex roots come in conjugate pairs, what else must necessarily be a zero? So what else is a factor?
Multiply these two factors together. Then divide this quadratic out of the fourth-degree polynomial. Apply the Quadratic Formula to the result to find the other two roots.
Eliz.