At a certain factory, the total of manufacturing q units is C(q)= 0.3q^(2)+q+300 dollars. It has been determined that approximately q(t)=t^(2)+150t units are manufactured during the first t hours of a production run. Compute the rate at which the total manufacturing cost is changing with respect to time 3 hours after productions commences.
After 3 hours the manufacturing cost is changing at the rate of $________ per hour .

Please, I don't understand how they do it!

Thanks!! ;)

Question

At a certain factory, the total of manufacturing q units is C(q)= 0.3q^(2)+q+300 dollars. It has been determined that approximately q(t)=t^(2)+150t units are manufactured during the first t hours of a production run.  Compute the rate at which the total manufacturing cost is changing with respect to time 3 hours after productions commences.
After 3 hours the manufacturing cost is changing at the rate of $________ per hour .

Please, I don't understand how they do it! 

Thanks!! ;)