2x+5y+4z=0-----＞ 6x+15y+12z=0 (1)
3x+y-7z=0-----＞6x=2y-14z=0 (2)
(1)(2)式消去6x,得:
y+2z=0 (3)
代入x+y-z,得:x-3z=?
(3)代入(2),得:
x-3z=0
所以x+y-z=0