Disposable income is the amount available for spending and saving after taxes have been paid and is one gauge for the state of the economy. Using U.S. Energy Administration data for selected years...

Disposable income is the amount available for spending and saving after taxes have been paid and is one gauge for the state of the economy. Using U.S. Energy Administration data for selected years from 2010 and projected to 2040, the total U.S. disposable income, in billions, can be modeled by

wheret is the number of years past 2010. In what year is disposable income predicted to be $18 trillion (that is, $18,000 billion)?