The number (in billions) of pieces of mail handled by the U.S. Post Office each year from 2010 through 2019 can be approximated by P(t) = -0.003584t⁴ - 0.01051t³ + 0.7432t² - 6.508t + 171.7 where t is the number of years since 2010. Find and interpret the rate of change in the volume of mail for the following years.

(a) 2010
(b) 2018