9-x²-4y²+4xy
=9-(x^2+4y^2-4xy)
=9-(x-2y)^2
=(3+x-2y))*(3-x+2y)
x²-4y²+16y-16
=x^2-(4y^2-16y+16)
=x^2-(2y-4)^2
=(x+2y-4)*(x-2y+4)