(3/x^2-9)-(7/x^2+4x-21)=-4/x^2-4x+12
3/(x^2-9)-7/(x^2+4x-21)
=3/(x+3)(x-3)-7/(x+7)(x-3)
=[3(x+7)-7(x+3)]/(x+3)(x-3)(x+7)
=-4x/(x+7)(x+3)(x-3)