5 Find the value of \(a\) for which the system of equations
$$\begin{aligned}
& x - y + 2 z = 4
& x + a y - 3 z = b
& x - y + 7 z = 13
\end{aligned}$$
where \(a\) and \(b\) are constants, has no unique solution.
Taking \(a\) as the value just found,
- find the general solution in the case \(b = - 5\),
- interpret the situation geometrically in the case \(b \neq - 5\).