[(x-y)²-(x+y)(x-y)]÷(-2y)=[(x-y)²-(x²-y²)]÷(-2y)=[(x²-2xy+y²)-(x²-y²)]÷(-2y)=(x²-2xy+y²-x²+y²)÷(-2y)=(2y²-2xy)÷(-2y)=2y(y-x)÷(-2y)=x-y
