x^2-y^2-z^2+2yz =x^2-(y-z)^2=(x+y-z)(x-y+z)
=x^2-(y-z)^2=(x-y+z)(x+y-z)
x2-y2-z2+2yz =x2-(y-z)2=(x+y-z)(x-y+z)
