A manufacturer finds that the cost of producing x units weekly is

C(x)=3000-55x+4x^2 dollars

If the production level is increasing steadily at the rate of 5 units per week. how fast are production cost changing when the production level reaches 8 units per week.

So i first took the derivative but am stuck of what to do next?

$\displaystyle dc/dt = -55dx/dt + 8dx/dt$