Answer
$\left[\begin{array}{lll}
4 & \fbox{$9$} & -7\\
7 & -7 & \fbox{$0$}\\
\fbox{$4$} & -5 & -5
\end{array}\right]$
Work Step by Step
$AB=C$
$c_{ij}=$the inner product of (row $i$ of A) times (column $j$ of B)
----
The missing entry in $c_{12}$ row 1, column 2,
is the inner product of (row 1 of A) times (column 2 of B)
$c_{12}=3(3)+1(-2)+2(1)=9$
$c_{23}=-1(-2)+2(-1)+0(0)=0$
$c_{31}=1(-1)+3(3)-2(2)=4$
