Given the following system of equations $\left\{ \begin{array} { l } 2 x + a y + z = a \\ x - 4 y + ( a + 1 ) z = 1 \\ 4 y - a z = 0 \end{array} \right.$
a) (2 points) Discuss depending on the values of the real parameter $a$.
b) ( 0.5 points) Solve the system for $a = 1$.
c) ( 0.5 points) Solve the system for $a = 2$.