已知x,y,z是三个非负有理数,且满足3x+2y+z=5,x+y-z=2,若S=2x-y-z,求s的取值范围

∵3x+2y+z=5,x+y-z=2
∴x=1-3z,y=1+4z
∴S=2x-y-z
=2(1-3z)-(1+4z)-z
=1-11z
又x,y,z是非负有理数,即z>0
∴1-11z-8/3
∴S>-8/3 ②
又z=(y-1)/4
∴S=1-11z
=(15-11y)/4
∵y>0
∴（15-11y)/4