Relative extrema question

**Find the x-values of all points where the functions defined as follows have any relative extrema. Find the value(s) of any relative extrema.**

f(x)= 3x^(5/3) - 15x^(2/3)

I did diff(3*x^(5/3)-15*x^(2/3), x) = 5*x^(2/3)-10/x^(1/3)

solve(5*x^(2/3)-10/x^(1/3) = 0)

x = 2

y = 3*(2)^(5/3) - 15*(2)^(2/3) = - 14.28660947

Vertex = (2, - 14.28660947)

f''(x) = 10/(3x^(1/3)) + 10/(3x^(4/3))

f''(2) = 10/(3(2)^(1/3)) + 10/(3(2)^(4/3))

˜ 3.97

So f(2)= -14.286 is my relative minimum[COLOR=rgb(0, 0, 0)]**?**[/COLOR]