November and December retail sales, excluding autos, for the years 2001-2009 can be modeled by the function
S(x) = -1.681x² + 38.749x +389.391 billion dollars, where x is the number of years after 2000.
a. Graph the function for values of x representing 2001-2009,
b. During what years does the model estimate the sales to be $550 billion?