3a(x-y)+9(y-x)
=3(x-y)(a-3)
x^2+y^2-1-2xy
=(x-y)^2-1
=(x-y+1)(x-y-1)
(x+2)(x+6)+x^2-4
=(x+2)(x+6)+(x+2)(x-2)
=(x+2)(x+6+x-2)
=(x+2)(2x+4)
=2(x+2)^2