x2 - y2 - 2yz - z2
Using the formula
[∵ (x - y)2 = x2 + y2 - 2xy]
= x2 - (y2 + 2yz + z2)
= x2 - (y + z)2
= (x)2 - (y + z)2
Using the formula
[∵ (x2 - y2) = (x + y)(x - y)]
= (x - (y + z))(x + (y + z))
= (x - y - z)(x + y + z)
Hence, x2 - y2 - 2yz - z2 = (x - y - z)(x + y + z)