1.
9x2 - 6xy + y2 - (2xy)2
Using a bracket to seperate them
= (9x2 - 6xy + y2) - (2xy)2
Using the identity of (a - b)2 ≡ a2 - 2ab + b2 to factorize the first part
= (3x - y)2 - (2xy)2
Using the identity of a2 - b2 ≡ (a + b)(a - b)
= [(3x - y) + 2xy][(3x - y) - 2xy]
= (3x + 2xy - y)(3x - 2xy - y)