3x^2-11xy+6y^2-xz-4yz-2z^2=(3x^2-11xy+6y^2)+(2xz-6yz)-(3xz+2yz)-2z^2=(3x-2y)(x-3y)+2z(x-3y)-z(3x-2y)-2z^2=(x-3y)(3x-2y+2z)-z(3x-2y+2z)=(3x-2y+2z)(x-3y-z)