①x^4-4x^3y+4x^2y^2
=x^2(x^2-4xy+4y^2)
=x^2(x-2y)^2
②(2x+3y)^2-(2x-y)^2
=[(2x+3y)+(2x-y)][(2x+3y)-(2x-y)]
=(4x+2y)(4y)
=8y(2x+y)