Have a few questions guys

1. Test for concavity and inflection points.

**So, to find the inflection points, I know I need to apply the 2nd derivative test which I got:

From here, I set the 2nd derivative =0 which I am having a hard time with.... I think I need to factor somewhere but I don't see it (well its not obvious to me hence I am here. ) For Concavity, you just plug in numbers that are bigger or smaller than your critical # to find where its concavity correct?

And can someone help me with the concavity question?

The Second question:

2. Test the function for relative maximum and minimum.

** I applied the second derivative test again to try and find the critical points: f''(x)=24x-216

Which my textbook tells me is wrong.

As usual, TIA