x + y + z = 9 ----- eq (i)
- x + y + z = 1 ----- eq (ii)
x - y - z = 5 ----- eq (iii)
All you have to do here is ADD eqs (ii) & (iii). All 3 variables will be ZEROED out on the left, leaving the equation: 0 = 6.
Obviously, this is FALSE so no solutions exist ().