2:{ x-4y+z =-3①2x+y-z = 18②x-y-z = 7③
3:{ 2x+3y+z = 6①x-y+2z =- 1②x+2y-z = 5③
4:{ 3x+2y+z = 13①x+y+2z = 7②2x+3y-z = 12③
The first question is contradictory.
y+z=0,y+z= 1
the second question
{x-4y+z=-3 ①
2x+y-z= 18 ②
x-y-z=7 ③
①+②: x-y=5 ④
①+③: 2x-5y=4 ⑤
④×2-⑤: 3y=6。
∴y=2,x=7
∴z=-2
Third question
2x+3y+z=6 ①
x-y+2z=- 1 ②
x+2y-z=5 ③
①+③ De: 3x+5y= 1 1 ④
②+③×2: 3x+3y=9 ⑤
④-⑤: 2y=2
∴y= 1,x=2
∴z=- 1
The fourth question
3x+2y+z= 13 ①
x+y+2z=7 ②
2x+3y-z= 12③
①+③ De: 5x+5y=25 ④
②+③×2: 5x+7y=3 1 ⑤。
⑤-④ Score: 2y=6
∴y=3,x=2
∴z= 1