$7-4.$ Consider the two$-$node circuit shown in Fig. P$7\mathrm{.}4.$ The voltages $V_1$ and $V_2($in volts$)$ satisfy the following system of equations:

$4V_1-V_2=20$

$-3V_1+8V_2=40$

$($a$)$ Find $V_1$ and $V_2$ using the substitution method.

$($b$)$ Write the system of equations in the matrix form $AV=b,$ where $V=\left[\begin{array}{c}{V}_1\\ V_2\end{array}\right].$