Degree 3 = Three zeroes.
These zeroes (say a, b, and c) are expressed as factors in a polynomial in the form
f(x) = (x-a)(x-b)(x-c)
I see you already have two zeroes, and need a third one to complete this polynomial. Since you need a polynomial with real coefficients, you must make sure the imaginary part of the zero (3 + i) is eliminated during the expansion.
Hint: Any complex zeroes will exist as conjugate pairs in polynomial with real coefficients (conjugate pair theorem).