so I tried doing this problem but I'm having difficulty, please help

find all critical numbers for the function f(x)=(9-x^2)^(3/5)

First thing i did was to find the derivative which i got -6x/[5(9-x^2)^(2/5)]

The next thing was to find when f'(x)=0, so i set the derivative equal to 0 to get x=0

I then checked to see when the derivative is undefined. To do this i set the denominator of the derivative [5(9-x^2)^(2/5)] to zero and got x=3 and x=-3.

are x=0,-3,3 right? I graphed the original function on my calculator and can't find anything specific about these points. also, i used the derivative function on my calculator to get the derivatives at these points, i thought that i'm not supposed to be able to do this since the derivative is supposed to be undefined at x=3 and x=-3