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