Note: I couldn't get fractional exponents to show up properly, so I used this key:

We need to find where f is concave up and down. There's two inflection points, one of which I've (correctly) found to be (0,0), which means that f is concave up at and from and it's concave down from .

But f''(x) never goes negative, and it's undefined at 0, so I'm not sure how to find s.