解
(9y²-x²)/(x²+6xy+9y²)÷(x-3y)/(x²+3xy)
=(3y-x)(3y+x)/(x+3y)²÷(x-3y)/x(x+3y)
=-(x-3y)/(x+3y)×x(x+3y)/(x-3y)
=-x