Four months ago, Cynthia signed up to receive email notifications to remind her to change her contact lenses,
Cynthia's first email arrived 9 days after she set up the notifications, and her second email arrived 14 days after
the first. Emails continued to arrive every 14 days.
Assuming n is a positive integer, which function gives the number of days, c(n), after Cynthia set up the
notifications that her nth email arrived?