The average cost of printing a book in a publishing company is c(x) = 5.5x+kx , where x is the number of books printed that day and k is a constant. Find k, if on the day when 200 were printed the average cost was $9 per book.

We are given: c(200) = 9, so we have:
9 = 5.5(200) + k(200)
200k + 1100 = 9