(a) Show that the system of equations
\(\begin{array}{l} x-y+2 z=4 \\ x-y-3 z=a \\ x-y+7 z=13 \end{array}\)
where \(a\) is a constant, does not have a unique solution.
(b) Given that \(a=-5\), show that the system of equations in part (a) is consistent. Interpret this situation geometrically.
(c) Given instead that \(a \neq-5\), show that the system of equations in part (a) is inconsistent. Interpret this situation geometrically.