The marginal cost of printing a poster when x posters have been printed

Question

The marginal cost of printing a poster when x posters have been printedThe marginal cost of printing a poster when x posters have been printed is
dc
dx
21X
dollars. Find c(25) – c(9), the cost of printing posters 10-25.
The cost of printing posters 10-25 is
dollars.
(Simplify your answer.)Math