I have looked every way in this, tried some identities, substitutions etc. and always came close to something but never found a winning combination. I am sure it is a pretty simple substitution, I just don't see it.

$\displaystyle \int x^2 \cdot \text{tanh}^2 \ (x^3) \cdot dx$

I thought of setting $\displaystyle u = \text{cosh} \ (x^3)$ but it would give me $\displaystyle 2 \cdot \int \frac{\text{sinh} \ (x^3)}{u^2} \cdot du$ which I am still unable to resolve. (I also managed to get something similar but divided by $\displaystyle u$ instead of $\displaystyle u^2$ with identities, but it is worthless since I cannot resolve it either.)

Like I said, I know it is really simple and I just don't see it. Thanks for helping me out.