As I posted yesterday (when the value of 'm' was not given in the original posting) , the best mileage will occur at speed , s = - b/2a where b = 3+m = 11 and a = - 1/28
- 11 / ( 2* (- 1/28)) = 154 mi/hr ( not very realistic....but that is the answer given the equation in the question)
substituting this value into the equation to find the mileage results in: (-1/28) ( 154^2) + 11(154) + 39 = 886 m/gal ( again, not very realistic)