please need help solving:
1/((s^2)+1)^3
You mean you wish to show:
$\displaystyle \mathbb{L}^{-1}\left\{\frac{1}{(s^2+1)^3}\right\}=1/8\left(3\sin(t)-3t\cos(t)-t^2\sin(t)\right)$
We can break it up into partial fractions I would guess and then take the inverse transform of each. Haven't done so yet. Can you do that?