The fuel efficiency F for a particular car is given by F(x) = -0.016 x2 + 1.44 x + 5.4 where x is the speed of the car in miles per hour (mph) and F(x) is the corresponding fuel efficiency in miles per gallon (mpg).
(a) What is the fuel efficiency if the car's speed is 70 mph?
(b) What speed will yield the maximum fuel efficiency? Show work.