Answer
Row by row, the missing entries are
$p_{12}$ =$9$
$p_{23}$ = $0$
$p_{31}$ = $4$
Work Step by Step
If $A$ is an $m\times n$ matrix and $B$ is an $n\times k$ matrix
(so the number of columns of $A$ is the same as the number of rows of $B$),
then the matrix product $AB$ is the $m\times k$ matrix
whose $ij$-entry is the inner product of the $i\mathrm{t}\mathrm{h}$ row of $A$ and the jth column of $B.$
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Naming the product matrix $P=[p_{ij}],$
$p_{12}$ = (row 1 in A) times (column 2 in B)$=3(3)+1(-2)+2(1)=9$
$p_{23}$ = (row $2$ in A) times (column $3$ in B)$=-1(-2)+2(-1)+0(0)=0$
$p_{31}$ = (row $3$ in A) times (column $1$ in B)$=1(-1)+3(3)+(-2)(2)= 4$
