Answer
$a.\quad \left[\begin{array}{rrr}
5 & -7/2 & 15\\
3 & -7 & 15
\end{array}\right]$
$ b.\quad$ not defined
Work Step by Step
$a.$
$3B+2C=3\left[\begin{array}{lll}
3 & 1/2 & 5\\
1 & -1 & 3
\end{array}\right]-2\left[\begin{array}{lll}
2 & -5/2 & 0\\
0 & 2 & -3
\end{array}\right]$
$=\left[\begin{array}{lll}
9 & 3/2 & 15\\
3 & -3 & 9
\end{array}\right]-\left[\begin{array}{lll}
4 & -5 & 0\\
0 & 4 & -6
\end{array}\right]$
$=\left[\begin{array}{lll}
5 & -7/2 & 15\\
3 & -7 & 15
\end{array}\right]$
$b.$
$2H$ is a 2$\times$2 matrix, $D$ is a 1$\times$2 matrix.
Their sum is not defined, since their dimensions are different.
