I assume you found that
We want the second derivative. Do not expand anything at this stage, just differentiate. We get, using the Quotient Rule,
Again, do not expand, look instead for common factors in the numerator. The numerator is equal to
which simplifies to $(x^2+3)(6)(3-3x^2)$.
The last simplification (cancelling) is not really necessary or even useful, and sometimes can be a bad idea. The reason is that when you use the Quotient Rule, the denominator is naturally a square, so is $\ge 0$, and can usually be forgotten about if we are interested only in the sign of the derivative. So it is usually a good idea to leave the natural denominator alone.
I have not checked whether your expression for the second derivative is correct. But there are two things to consider before expanding. Any calculation carries some risk of error. And after expanding, there is a good chance that you will have to factor in order to find out where your expression is positive, negative, or $0$. After the expansion, the factorization may be far from obvious.
Remark: Scrambling an egg is far easier than unscrambling it. When an expression has structure, it is useful to hang on to that structure as much as possible.