令x=2k,y=3k,z=4k
(x-y-z)/(3x+2y-z)
=(2k-3k-4k)/(3*2k+2*3k-4k)
=-5k/8k
=-5/8
2x+3y-4z=-3
2*2k+3*3k-4*4k=-3
-3k=-3
k=1
3x-4y-z
=3*2k-4*3k-4k
=-10k
=-10*1
=-10