y=x^5/3-x^2/3

y'=(5/3)x^2/3-(2/3)x^(-1/3)

To make y'=0 , we have

5x^2/3=2x^(-1/3)

(x^2/3)(x^1/3)=2 /5

x=2/5

That is, y has an extreme value when x-2/5. At this point,

y=(2/5-1)(2/5)^2/3

≈ - 0.3257