The annual revenue of Amazon over the period 2006-2010 can be approximated by a ( t ) = 2 e 0.25 t b i l l i o n d o l l a r s p e r y e a r ( 6 ≤ t ≤ 10 ), where t is the time in years since the start of 2000. Find an expression for the total annual revenue A ( t ) earned since the start of 2006, and use it to estimate the total revenue earned from the start to 2006 to the start of 2009. Round answer to the nearest $ billion. Do not include a dollar sign with your answer below. Hints: A ′ ( t ) = a ( t ) and A ( 6 ) = 0. Why?