（x-2y)(x-2y-3）-x(x+4y-3)
=(x-2y)[(x-2y)-3]-x^2-4xy+3x
=(x-2y)^2-3(x-2y)-x^2-4xy+3x
=x^2-4xy+4y^2-3x+6y-x^2-4xy+3x
=4y^2+6y.