Answer
(a) $C_{23}=32$.
(b) $C_{32}=13$.
Work Step by Step
Given $$
A=\left[\begin{array}{rrr}{4} & {11} & {-9} \\ {0} & {3} & {2}\\ {-3} & {1} & {1}\end{array}\right], \quad \text { and } \quad B=\left[\begin{array}{rrr}{1} & {0} & {5} \\ {-4} & {6} & {11}\\ {-6} & {4} & {9} \end{array}\right].
$$
We have
\begin{align*}
C=5A+2B&=5\left[\begin{array}{rrr}{4} & {11} & {-9} \\ {0} & {3} & {2}\\ {-3} & {1} & {1}\end{array}\right]+2\left[\begin{array}{rrr}{1} & {0} & {5} \\ {-4} & {6} & {11}\\ {-6} & {4} & {9} \end{array}\right]\\
&=\left[\begin{array}{rrr}{20} & {55} & {-45} \\ {0} & {15} & {10}\\ {-15} & {5} & {5}\end{array}\right]+\left[\begin{array}{rrr}{2} & {0} & {10} \\ {-8} & {12} & {22}\\ {-12} & {8} & {18} \end{array}\right]\\
&=\left[\begin{array}{rrr}{22} & {55} & {-35} \\ {-8} & {27} & {32}\\ {-27} & {13} & {23} \end{array}\right].
\end{align*}
(a) $C_{23}=32$.
(b) $C_{32}=13$.
