Stuck on PART C, PART D, and PART E. Any help would be appreciated so I can figure out why I'm getting this wrong.

I have the function f(x)= x + 12x^(2/3)

PART A: First I found the domain (-infinity, infinity). This was confirmed correct by the system.

PART B: Next I am supposed to find the horizontal and vertical asymptotes of f(x). I got NA for both. This was confirmed correct by the system.

PART C: Next, I am supposed to find the intervals of increase and decrease of f(x). For increasing I got (0, infinity) and for decreasing I got (-infinity, -512)U(-512, 0) - and both of these were marked wrong. Not sure what I'm doing wrong with this part.

PART D: Find the local maximum and minimum values of f(x).

I got some parts of this one right and some wrong and I'm not sure why. So I'll put ??? where I am not sure why my answer was wrong and fill out what I did get correct.

f(???) = ??? is the local max

f(???) = 0 is the local min

PART E: Give the intervals of concavity.

For concave upward I got NA, which was marked correct, but for concave downward I got (-infinity, infinity) which was marked incorrect and I'm not sure why.