[(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