[（x的平方＋y的平方）－（x－y）的平方＋2y（x－y）]÷4x
=(x²+y²-x²+2xy-y²+2xy-2y²)÷4x
=(4xy-2y²)÷4x
=2y(2x-y)÷4x
=-3y÷x
=-3(2x+6)÷x
=-6-18/x