Describe the concavity of the graph off(x = 3(x4)^5/3and find the points of inflection (if any).

I know how to find concavity with the 2nd derivative. I got the 2nd derivative but the way the function is, confuses me. I set my 2nd derivative to zero but it is a fraction with sq roots.. How do I get the CP value from there??Please show step by step and I will rate best answer!