b
(b) $A = \left[ {\,\begin{array}{*{20}{c}}{ - 1}&2\\2&{ - 1}\end{array}\,} \right]\,\,,\,\,\,B = \left[ \begin{array}{l}3\\1\end{array} \right]$
$AX = B\, \Rightarrow X = {A^{ - 1}}B$
${A^{ - 1}} = \frac{{adj\,A}}{{|A|}}$
${A^{ - 1}} = \frac{{ - 1}}{3}\left[ {\,\begin{array}{*{20}{c}}{ - 1}&{ - 2}\\{ - 2}&{ - 1}\end{array}\,} \right]$ $ = \frac{1}{3}\left[ {\,\begin{array}{*{20}{c}}1&2\\2&1\end{array}\,} \right]$
and $X = {A^{ - 1}}B$ = $\left[ {\,\begin{array}{*{20}{c}}{\frac{1}{3}}&{\frac{2}{3}}\\{\frac{2}{3}}&{\frac{1}{3}}\end{array}\,} \right]\,\left[ \begin{array}{l}3\\1\end{array} \right]$; $X = \frac{1}{3}\left[ \begin{array}{l}5\\7\end{array} \right]$.