4-4x^2+4xy-y^2=4-(4x²-4xy+y²)=2²-(4x-y)²=(2-4x+y)(2+4x-y)-4xy+4y^2-2x+4y+1=4y²+4y+1-4xy-2x=(2y+1)²-2x(2y+1)=(2y+1)(2y-2x+1)