Time rate of change of cost problem

The cost of manufacturing *x* cases of umbrellas is *C* dollars (in thousands), where http://www.webassign.net/cgi-bin/sym...x%29%20%2B%209

Monthly production at *t* weeks is expected to be http://www.webassign.net/cgi-bin/sym...20%2B%2080%20t

I know that http://www.webassign.net/cgi-bin/sym...?expr=C%28t%29 = 18(5000+80t)+4sqrt(5000+80t)+9

and that marginal cost or http://www.webassign.net/cgi-bin/sym...%29%2F%28dx%29 = 28+(2/x^.5)

I'm not sure how to find http://www.webassign.net/cgi-bin/sym...%29%2F%28dt%29 in order to solve the equation for t=2 in order to find how fast costs are rising per week.