1.4. Find the value of the polynomial $f(A)$ of the matrix $A$:

$$
f(x)=x^{2}-5 x, A=\left(\begin{array}{cc}
1 & -1 \\
2 & 3
\end{array}\right)
$$
$$
\begin{aligned}
f(A) & =\left(\begin{array}{cc}
1 & -1 \\
2 & 3
\end{array}\right)^{2}-5\left(\begin{array}{cc}
1 & -1 \\
2 & 3
\end{array}\right)=\left(\begin{array}{cc}
1 & -1 \\
2 & 3
\end{array}\right) \cdot\left(\begin{array}{cc}
1 & -1 \\
2 & 3
\end{array}\right)-5\left(\begin{array}{cc}
1 & -1 \\
2 & 3
\end{array}\right)= \\
& =\left(\begin{array}{cc}
(1 \cdot 1+(-1) \cdot 2) & (1 \cdot(-1)+(-1) \cdot 3) \\
(2 \cdot 1+3 \cdot 2) & (2 \cdot(-1)+3 \cdot 3)
\end{array}\right)-\left(\begin{array}{cc}
5 & -5 \\
10 & 15
\end{array}\right)= \\
& =\left(\begin{array}{cc}
-1 & -4 \\
8 & 7
\end{array}\right)-\left(\begin{array}{cc}
5 & -5 \\
10 & 15
\end{array}\right)=\left(\begin{array}{cc}
-1-5 & -4+5 \\
8-10 & 7-15
\end{array}\right)=\left(\begin{array}{cc}
-6 & 1 \\
-2 & -8
\end{array}\right) .
\end{aligned}
$$