Question: Matrix with two unknown parameters

We are a group of students trying to solve this, we can't find a way.

We are given the system of equations

ax + y +z +u =  1
  x  -y  -z +u  = -a
       y +z -au = b
  x- y +bz +u =-2a

For what values ​​of a and b, the system (the answer sould be in real, not complex).
a) A unique solution?
b) Infinitely many solutions?
c) No solution?

Here is the matrix in maple:

>with(Linear Algebra):

thank's for all help and fast answers

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