X^2-4xy+4y^2-(x-2y)^3
x^2-4xy+4y^2 = (x -2y)^2
x^2-4xy+4y^2-(x-2y)^3 =(x -2y)^2 - (x -2y)^3 = (x -2y)^2 * ( 1 -x+2y)