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
代入(5)
4*29/9+2z=15
z=19/18
代入(3)
29/9+y+19/18=12
y=139/18