Employment. According to the U.S. Bureau of Labor Statistics, the number of professional services employees Fluctuated during the period $2000-2009,$ as modeled by $E(t)=-28.31 t^{3}+381.86 t^{2}-1162.07 t+16,905.87$
where $t$ is the number of years since $2000(t=0$ corresponds to 2000 ) and $E$ is thousands of employees. Find the relative extrema of this function, and sketch the graph. Interpret the meaning of the relative extrema.