(x+y)²-(a+b)²
=(x+y+a+b)(a+y-a-b)

(x-y)²-(a-b)²
=(x-y+a-b)(x-y-a+b)

(xy)²-(ab)²
=(xy+ab)(xy-ab)

x²-6x+9-y²
=(x-3)²-y²
=(x-3+y)(x-3-y)