6x²-5xy-6y²-2xz-23yz-20z² 
=6x^2-(5y+2z)x-6y^2-23yz-20z^2 
=6[x^2-(5/6y+1/3z)x+(3/2y+2z)(-2/3y-5/3)z 
=6(x-3/2y+2z)(x+2/3y+5/3z) 
=(2x-3y+4z)(3x+2y+5z)