a) x2 - y2 - z2 - 2yz
=x2 - (y2 + 2yz + z2)
=x2 - (y + z)2
=(x - y - z)(x + y + z)
b)4x2(x - 6) + 9y2(6 - x)
=4x2(x - 6) - 9y2(x - 6)
=(x - 6)(4x2 - 9y2)
=(x - 6)(2x - 3y)(2x + 3y)
c)6xy + 5x - 5y - 3x2 - 3y2
=(-3x2 + 6xy - 3y2) + (5x - 5y)
= -3(x2 - 2xy +y2) + 5(x - y)
= -3(x - y)2 + 5(x - y)
=(x - y)(-3x + 3y + 5)