The average cost of producing x units of a commodity is given by the equation

c(x) = 20.4 - 0.0007x. Find the marginal cost at a production level of 1255 units. Round to two decimal places.

It is my understanding that to figure out marginal cost, you take the derivative of c(x) and substitute 1255, correct? Well, differentiating in this case takes away all of the x's.....so I am completely lost. Is there no marginal cost?