Chlorine is frequently used to disinfect swimming pools. The chlorine concentration should remain between 1.5 and 2.5 parts per million. On warm sunny days with many swimmers agitating the water, 30% of the chlorine can dissipate into the air or combine with other chemicals each day.

(a) Find C and a so that f(x) = Ca^x models the amount of chlorine in the pool after x days. Assume that the initial amount is 2.5 parts per million. 

(b) What is the chlorine concentration after 2 days if no chlorine is added?