I think the responses so far are somewhat too bogged down in the details of computing the solution. Take a step back and look at the original problem, which is a business/economics question. Whate value of q minimizes the marginal cost? Standard calculus procedure: determine where (a)...
Since the trouble was originally that you got 0/0 by plugging 6 in directly, you could also have used L'Hopital's rule. Of course, it is conceptually more elementary to factor the polynomial, but it may actually be less work to do 1 derivative than to do the polynomial long division.
Just...
It's absolutely not possible. Fourier theory says that if any function has a Fourier series (or Fourier transform) at all, then that transform is unique. Since a dc signal can be written as exp(i*0*t), this is its expression in terms of Fourier components. No other expression can possibly...