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