x+3y+5z=0(1)
2x+3y+z=0(2)
(2)-(1)
x-4z=0
x=4z
代入(1)
4z+3y+5z=0
3y=-9z
y=-3z
所以x^2=16z^2,y^2=9z^2
(2y-x)(2y+x)/z^2
=(4y^2-x^2)/z^2
=(4*9z^2-16z^2)/z^2
=20z^2/z^2
=20