Answer
We get the matrix $\left[ \begin{matrix}
27 & 32 \\
-19 & -20 \\
\end{matrix} \right]$ which decodes to the message $8,5,12,16$, or HELP.
Work Step by Step
Consider the numerical value of $\text{HELP}$ as, $\begin{align}
& H=8, \\
& E=5 \\
& L=12 \\
& P=16 \\
\end{align}$
Using the cryptogram method we get, $\begin{align}
& A=\left[ \begin{matrix}
4 & -1 \\
-3 & 1 \\
\end{matrix} \right]\left[ \begin{matrix}
8 & 12 \\
5 & 16 \\
\end{matrix} \right] \\
& =\left[ \begin{matrix}
32-5 & 48-16 \\
-24+5 & -36+16 \\
\end{matrix} \right] \\
& =\left[ \begin{matrix}
27 & 32 \\
-19 & -20 \\
\end{matrix} \right]
\end{align}$
Therefore, expression of the matrix is $\left[ \begin{matrix}
27 & 32 \\
-19 & -20 \\
\end{matrix} \right]$.
