The total energy consumption (in quadrillion Btu's) for the United States can be approximated by the function
f(x) = -.000144x3 + .014151x2 + .1388x + 23.35
where x = 0 corresponds to the year 1970.
a) Find the energy consumption for 1990, 2000, and 2008.
b) Find the average rate of change in energy consumption between 2000 and 2008
c) At what rate was energy consumption changing in 2008?