it's been a really long time since my last math course (10 years) and need some help with calculus!

I have found the derivative of: 1/6 - (2000-x)/ 4(sqrt((2000-x)^2+600^2) but now I'm a bit lost.

Any help would be great!

The time required to walk to a bus stop is given by

What value of minimizes the amount of time taken? [Where represents the distance walked.]