Math Project: Modelling Functions

The image is fairly large so here is the URL to the assignment that I scanned:

http://img9.imageshack.us/img9/4811/img065h.jpg

These are the points plotted:

http://i83.photobucket.com/albums/j283/ta_lia/01-7.jpg

The are the points plotted with a linear function which I THINK is an accurate fit:

http://i83.photobucket.com/albums/j283/ta_lia/02-3.jpg

What I am having serious trouble with is the function and how to estimate/interpret the function. I have not encountered anything like this before and I am very confused.

Please help me! Thank you.