已知实数x,y,z满足x-y=5,z^2=-xy-y-9,求x-2y+3z的值

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

答案=8 把X=5+Y代入Z（平方）=-XY-Y-9会得到Z（的平方）=-(Y+3)的平方
所以两边只能都等0得Z=0 Y=-3 X=2能看懂么