The price per gallon is the slope of this line, rise over run, change in y over change in x.
[tex]p=\dfrac{\frac{35}{4} - \frac{21}{4}}{\frac 5 2 - \frac 3 2} = \dfrac{14/4}{2/2}=\dfrac{7}{2}[/tex]
Answer: 3.50
