1.1 #20:

Let x=r, y=2, and z=1. This gives

 3r - 2 = 4
  r - 7 = 5
-2r + 8 = 9

But then, r = 2, r = 12, and r = -1/2, must all be true.
Thus, there is no value of r that solves all three equations.