Answer:
f(x) = x^4 - 6x^3 + 18x^2 - 54x + 81
Explanation:
We have a zero of 3 with multiplicity 2, and
since 3i is a root, -3i must also be a root.
f(x) = ((x - 3)^2)(x - 3i)(x + 3i)
f(x) = (x^2 - 6x + 9)((x^2 + 9)
f(x) = x^4 + 9x^2 - 6x^3 - 54x + 9x^2 + 81
f(x) = x^4 - 6x^3 + 18x^2 - 54x + 81