Question 195764
y + z = 0
- x + y - z = 9
x - z = 4 

[ 0  1  1][0]
[-1  1 -1][9]
[ 1  0 -1][4]


[1/3 -1/3  2/3][0] [-1/3 ]
[2/3  1/3  1/3][9] [13/3 ]
[1/3 -1/3 -1/3][4] [-13/3]