Answer
$-10$
Work Step by Step
Multiply each term of the first complex number by each term of the second, then combine like terms and use the fact that $i^2=-1$ to obtain:
$$
(-3+i)(3+i)=-3(3+i)+i(3+i)
\\=-3(3)+(-3)(i)+i(3)+i(i)
\\=-9-3i+3i+i^2
\\=-9+(-1)
\\=-10
$$