Okay, what have you done? What happens if you try to solve x, y, and z? I suspect that you will get x, y, or z, whichever you solve for first, equal to a fraction. There will be one solution if the denominator is not 0, no solution if the denominator is 0 but the numerator is not, and an infinite number of solutions if both numerator and denominator are 0 . What values of a make each of those things happen?
(Look closely at the last two equations. Tthere is an obvious first step that will simplify things very fast!)