3/a+3-7/a-3+21/a^2-9

3/(a+3) - 7/(a-3)+21/(aˇ2-9)=3/(a+3)-7/(a-3)+21/(a+3)(a-3)=
=(3a-9-7a-21+21)/(aˇ2-9)=(-4a-9)/(aˇ2-9)