Ok, I really need someone to explain this to me

__step-by-step__, PLEASE! I have literally worked on this problem for three days!! Even just trying it would be appreciated!

**Gasoline mileage of automobiles is influenced by the horsepower of the automobile. Below is a table of the data, use the data table to answer the following question:** **HP(x) 49|52|55|58|61|64|67**

MPG(y) 65.4|56.5|46.0|49.2|40.3|39.4|36.1 **QUESTION: Find a linear model that fits the data, round to four decimal places**
Just an FYI, I know how to find linear models, but this one has gotten me completely confused. I know I need to start off with finding the

slope, which I've tried, and continued to get wrong (the answer is in the back of the book). Does it have something to do with the fact that the first difference isn't equally separated? Please Help!!