Answer
$a.\quad\left[\begin{array}{rrr}
-1 & -3 & -5\\
-1 & 3 & -6
\end{array}\right]$
$b.\quad\left[\begin{array}{rrr}
-6 & 16 & 10\\
2 & -14 & 24
\end{array}\right]$
Work Step by Step
$a.$
$C-B=\left[\begin{array}{lll}
2 & -5/2 & 0\\
0 & 2 & -3
\end{array}\right]-\left[\begin{array}{lll}
3 & 1/2 & 5\\
1 & -1 & 3
\end{array}\right]=\left[\begin{array}{lll}
-1 & -3 & -5\\
-1 & 3 & -6
\end{array}\right]$
$b.$
$2C-6B=2\left[\begin{array}{lll}
3 & 1/2 & 5\\
1 & -1 & 3
\end{array}\right]-6\left[\begin{array}{lll}
2 & -5/2 & 0\\
0 & 2 & -3
\end{array}\right]$
$=\left[\begin{array}{lll}
6 & 1 & 10\\
2 & -2 & 6
\end{array}\right]-\left[\begin{array}{lll}
12 & -15 & 0\\
0 & 12 & -18
\end{array}\right]$
$=\left[\begin{array}{rrr}
-6 & 16 & 10\\
2 & -14 & 24
\end{array}\right]$
