y = 68950e-0.2668x

I'm not quite sure what you need to do, but I threw the numbers on Excel, and that's the best fit curve.

I haven't a clue what kind of class you're taking, but this wiki link will provide some basic info on geometric progressions and links to exponential growth.

Edit: Thinking about it some, there may be some iterative method to finding the equation of the curve (if that's what you need to do), but it's been a few years since I've done that stuff.