The number of U.S. forces in Afghanistan decreased to approximately 34,000 in 2014 from a high of about 100,000 in 2010. The amount of U.S. funding for Afghan security forces also decreased during this period. The function f(x) = -1.321x² + 5.156x + 5.517 can be used to estimate the amount of U.S. funding for Afghan security forces, in billions of dollars, x years after 2009. Use this function for Exercises.

In what year was the amount of U.S. funding for Afghan security forces about $10.5 billion?