The given formula that models the number of songs in Martin's iTunes collection is:
Where m is the number of months.
We can interpret the terms in this formula as: 200 is the current number of songs in Martin's collection, and 15 is the number of new songs Martin adds every month.
We can prove this by replacing m=1, so:
So, in the first month, Martin will have 215 songs, for a total of 15 new songs.
The answer is: Every month, Martin adds 15 new songs.