Stuff like this is why people hate math. You've done your first derivative wrong - make sure you're doing the chain rule correctly.
Also, your Leibnitz notation is wrong. It's
, not
.
What are you asking for exactly? The answer itself? I'm not convinced you know what "inflection point", "concave up", and "concave down" even mean.
Spoiler
Also, your Leibnitz notation is wrong. It's
What are you asking for exactly? The answer itself? I'm not convinced you know what "inflection point", "concave up", and "concave down" even mean.

