7 mi/gal
From your graph you can see that the relationship of your problem is:
y=7x the number 7 is the slope of your line (=inclination) and tells you that for every gallon you can make 7 miles. This is your unit rate.
You can get to this number observing your two points P2(5,35) and P1(1,7); you can evaluate the unit rate by doing:
Δy/Δx=(35-7)/(5-1)=28/4=7
