# Math Help - Critical numbers help

1. ## Critical numbers help

Okay, this is my problem:

I solved, and got the derivative of f '(x)= -4/5x(9-x^2)^2-5

Can someone help me and let me know If I got the correct derivative, and if so, where do I go from here? What do I set equal to zero to get the criticals?

2. Originally Posted by thatonegurl
Okay, this is my problem:

I solved, and got the derivative of f '(x)= -4/5x(9-x^2)^2-5

Can someone help me and let me know If I got the correct derivative, and if so, where do I go from here? What do I set equal to zero to get the criticals?
I think the correct derivative is..

1.(9-X^2)^0.6
2.(0.6)(-2X) multiplied by (9-X^2)^-0.4
3.-1.2X(9-X^2)^(-0.4)