x ²+9y ²+4z ²-6xy-12yz+4xz
=(x-3y)^2+4z^2-4z(3y-x)
3y=x+2z
x-3y=-2z
3y-x=2z
4z^2+4z^2-4z*2z
=8z^2-8z^2
=0