a) 2xy + 3zy + 6y + xz
= (2xy + 6y) + (xz + 3zy)
= 2y(x + 3y) + z(x + 3y)
= (x + 3y)(2y + z)
b) x2 - 10x + 25
= x2 - 2.x.5 + 52
= (x - 5)2
c) x2 + 6x + 9 - y2
= x2 + 2.x.3 + 32 - y2
= (x - 3)2 - y2
= (x - 3 - y)(x - 3 + y)
d) x3 - 4x2 - xy2 + 4x
= x(x2 - 4x + 4 - y2)
= x[(x - 2)2 - y2]
= x(x - 2 - y)(x - 2 + y)