a²–x²+4ab+4xy+b²–4y²

a²+4ab+b²-(x²–4xy+4y²)

a²+2ab+b²-6ab-(x-2y)²

(a+b)²-(x-2y)²-6ab

[(a+b)-(x-2y)][(a+b)+(x-2y)]-6ab