x-y=5
x=5+y
z^2=-xy-y-9=-(5+y)y-y-9=-y^2-6y-9=-(y+3)^2
所以,z=0,y+3=0
z=0,y=-3
x=5+y=5-3=2
x-2y+3z=2-2*(-3)+0=8