Otal personal income in the United States (in billions of dollars) for selected years from 1960 and projected to 2018 follows. Year 1960 1970 1980 1990 2000 2008 2018 Personal Income 411.5 838.8 2307.9 4878.6 8429.7 12,100.7 19,129.6 (a) These data can be modeled by an exponential function. Write the equation of this function, with x as the number of years past 1960. (Round your numerical values to four decimal places.)