(a) Evaluate the matrix product, $\mathrm{Ax}$, where
$$
\mathbf{A}=\left[\begin{array}{ll}
7 & 5 \\
1 & 3
\end{array}\right] \text { and } \quad \mathbf{x}=\left[\begin{array}{l}
x \\
y
\end{array}\right]
$$
Hence show that the system of linear equations
$$
\begin{array}{r}
7 x+5 y=3 \\
x+3 y=2
\end{array}
$$
can be written as $\mathbf{A x}=\mathbf{b}$ where $\mathbf{b}=\left[\begin{array}{l}3 \\ 2\end{array}\right]$.
(b) The system of equations
$$
\begin{array}{r}
2 x+3 y-2 z=6 \\
x-y+2 z=3 \\
4 x+2 y+5 z=1
\end{array}
$$
can be expressed in the form $A x=b$. Write down the matrices $A, x$ and $b$.