化简：（x-2y)(x-2y-3）-x(x+4y-3不建议傻算.告诉计算理由

（x-2y)(x-2y-3）-x(x+4y-3)
=(x-2y)(x-2y-3)-[(x-2y)+2y][(x-2y-3)+6y]
=(x-2y)(x-2y-3)-[(x-2y)(x-2y-3)+2y(x-2y-3+3x-6y)+12y^2]
= -2y(4x-8y-3)-12y^2
= 4y^2-8xy+6y

(x-2y)(x-2y-3)-x(x+4y-3)
=(x-2y)²-3(x-2y)-x²+4xy+3x
=(x-2y)²-3x+6y-x²+4xy+3x
=(x-2y)²-(x²-4xy+4y²)+4y²+6y
=(x-2y)²-(x-2y)²+4y²+6y
=4y²+6y

（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.