【x²+y²-9-2xy】/【x²-y²+6x+9】
=【(x²-2xy+y²)-9】/【(x²+6x+9)-y²】
=【(x-y)²-9】/【(x+3)²-y²】
=【(x-y+3)(x-y-3)】/【(x+3+y)(x+3-y)】
=【(x-y+3)(x-y-3)】/【(x+3+y)(x-y+3)】
=(x-y-3)/(x+3+y)
【x²+y²-9-2xy】/【x²-y²+6x+9】
=【(x²-2xy+y²)-9】/【(x²+6x+9)-y²】
=【(x-y)²-9】/【(x+3)²-y²】
=【(x-y+3)(x-y-3)】/【(x+3+y)(x+3-y)】
=【(x-y+3)(x-y-3)】/【(x+3+y)(x-y+3)】
=(x-y-3)/(x+3+y)