七下数学题答案``急``现在就要

3x-y+z=3 (1)
2x+y-3z=11 (2)
x+y+z=12 (3)
(1)+(2)
5x-2z=14 (4)
(1)+(3)
4x+2z=15 (5)
(4)+(5)
9x=29
x=29/9
z=(15-4x)/2=19/18
y=3x+z-3=139/18

5x-4y+4z=13 (1)
2x+7y-3z=19 (2)
3X+2y-Z=18 (3)
(1)+(3)*4
5x-4y+12x+8y=13+72
17x+4y=85 (4)
(2)-(3)*3
2x+7y-9x-6y=19-54
-7x+y=-35
7x-y=35 (5)
(4)+(5)*4
17x+28x=85+140
45x=225
x=5
y=7x-35=0
z=3x+2y-18=-3

3x-y+z=3 ---------------1
2x+y-3z=11 ---------------2
x+y+z=12 -------------------3
1*3+2
11x-2y=20------------4
2+3*3
5x+4y=47------------5
4*2+5
27x=87
x=87/27=29/9=3+2/9=29/9
y=13/2+11/9=7+13/18=139/18
z=19/9

2.5x-4y+4z=13 -----------1
2x+7y-3z=19 --------------2
3X+2y-Z=18---------3
2-3*3
-7x+y=-35-------------4
1+3*4
17x+4y=85----------5
5-4*4
45x=225
x=5
y=0
z=-3