Answer
(a) $C_{21}=-6$.
(b) $C_{13}=29$.
Work Step by Step
Given $$
A=\left[\begin{array}{rrr}{5} & {4} & {4} \\ {-3} & {1} & {2}\end{array}\right], \quad \text { and } \quad B=\left[\begin{array}{rrr}{1} & {2} & {-7} \\ {0} & {-5} & {1}\end{array}\right].
$$
We have
\begin{align*}
C=2A-3B&=2\left[\begin{array}{rrr}{5} & {4} & {4} \\ {-3} & {1} & {2}\end{array}\right]-3\left[\begin{array}{rrr}{1} & {2} & {-7} \\ {0} & {-5} & {1}\end{array}\right]\\
&=\left[\begin{array}{rrr}{10} & {8} & {8} \\ {-6} & {2} & {4}\end{array}\right]-\left[\begin{array}{rrr}{3} & {6} & {-21} \\ {0} & {-15} & {3}\end{array}\right]\\
&=\left[\begin{array}{rrr}{7} & {2} & {29} \\ {-6} & {17} & {1} \end{array}\right].
\end{align*}
Then,
(a) $C_{21}=-6$.
(b) $C_{13}=29$.
