｛6/(x^2-9)}-{2/x^2+6x+9}
=6/(x-3)(x+3)-2/(x+3)^2
=6(x+3)/(x-3)(x+3)^2-2(x-3)/(x-3)(x+3)^2
=(6x+18-2x+6)/(x-3)(x+3)^2
=(4x+24)/(x-3)(x+3)^2