(xx-yy-zz)(xx-yy-zz)-4yyzz
=(x^2-y^2-z^2)^2-(2yz)^2
=(x^2-y^2-z^2+2yz)(x^2-y^2-z^2-2yz)
=[x^2-(y-z)^2][x^2-(y+z)^2]
=(x+y-z)(x-y+z)(x-y-z)(x+y+z)