Calculus 2 - For which value(s) of "a" and "b" will the system of equations...?

[Balázs Kónya]

2x+y+3z=13,
x+y+z=6,
-x+y+az=b

have
i) no solution
ii) one solution
iii) infinitely many solutions. Give the solution set in this case.

Thanks a lot.