speed vs mpg

[ADP]

I am searching for an linear equation. I have been given various (9) MPH and corresponding MPG and my task is to determine the MPG based on a MPH that is not one of the 9 given. I have to also create a linear scatter diagram in Excel (which I already have given the 9 different MPH's and MPG's). After creating this diagram Excel spits out a y=0.21x+14.389 formula and a R2=0.5132, but I don't know what to do with this or even if this information is important. HELP! What is this formula and how do I do it? I STINK at math and have a learning problem with remembering math equations even after review. . .