To find the extrema, differentiate M: -0.03x+1.31=0. Solve for x=1.31/0.03=131/3=43.67 approx.
M(131/3)=21.30 mpg approx. M(30)=18.5 mpg, M(60)=17.3 mpg. So in the given range the maximum mpg is 21.30 mpg at speed 43.67 mph and the minimum is 17.3 mpg at 60 mph.