原式=(x-y)/(x+3y)-(x+3y)(x-3y)/(x+3y)²+2y/(x+3y)
=(x-y)/(x+3y)-(x-3y)/(x+3y)+2y/(x+3y)
=(x-y-x+3y+2y)/(x+3y)
=4y/(x+3y)
=-12/(2-9)
=12/7