Answer
$\color{blue}{\bf3-i}.$
Work Step by Step
RECALL:
If $a+bi$ is a zero of a polynomial function with a real number of coefficients, then its conjugate, $a-bi$, is also a zero of the function.
Thus, the missing zero of the given function is the conjugate of $3+i$, which is $\color{blue}{\bf3-i}.$