I can't help you with your answer, but I can tell you what you need to do.

Most problems with fitting to data arelinearbest fits to a data set. What you are looking to do is a quadratic best fit.

To get a quick answer, here will do. I recommend learning how to do it by hand. Given about 20 minutes I could show you how to get the results, but you might find that Googling quadratic regression might be more help.

-Dan