Hi, I don't understand how to figure out the result is "2008" for year. Can you help me please? Thank you!

The total amount spent by some number of people on clothing and footwear in the years 2000-2009 can be modeled by the quadratic function f(x) = - 4.136x2 + 71 .34x + 9707, where x = 0 represents January 1, 2000, x =1 represents January 1, 2001, and so on, and f(x) is in billions of dollars. According to the model, in what year during this period was the amount spent on clothing and footwear a maximum? f ..... '\\ In the year 2008', $ 405 billion was spent on clothing and footwear. (Round the nal answer for the amount spent to the nearest whole number as needed. Round all intermediate values to the nearest tenth as needed.)