Consider the following system of equations depending on the real parameter a:
$$\left. \begin{array} { l } a x - 2 y + ( a - 1 ) z = 4 \\ - 2 x + 3 y - 6 z = 2 \\ - a x + y - 6 z = 6 \end{array} \right\}$$
a) (2 points) Discuss the system according to the different values of $a$.\ b) ( 0.5 points) Solve the system for $a = 1$.