First it is needed to find the slope with the following equation
[tex]m=\frac{y2-y1}{x2-x1}[/tex]We take two points of the line. (10,50) and (3,15)
Then you substitute them on the equation
[tex]\frac{50-15}{10-3}=\frac{35}{7}=5[/tex]The answer would be 5 gal/min