b
$[\text { A.Adj }(\mathrm{A}) \cdot]\left(\mathrm{A}^{-1} \cdot \mathrm{A}\right)$
$ = |{\rm{A}}| \cdot {\rm{II}} \cdot {\rm{I}}$
$ = |{\rm{A}}| \cdot {\rm{I}}$
$ = 6{\rm{I}}$
$ = 2\left| {\begin{array}{*{20}{l}}
3&0&0\\
0&3&0\\
0&0&3
\end{array}} \right|$