The gross domestic product (GDP) of the United States, in trillions of dollars, from 2011 through 2015 is approximately
$$
G(t)=0.064 t^{2}+0.473 t+15.0 \quad(0 \leq t \leq 4)
$$
where $t$ is measured in years, with $t=0$ corresponding to $2011 .$ In constructing this model, the government used actual GDP figures from 2011 and 2012 and estimates for the years 2013 through 2015.
a. What was the U.S. GDP in 2011?
b. What is the predicted U.S. GDP for $2015 ?$