In 2013, worldwide cell phone subscribership's surpassed the world's total population of 6.8 billion. Using data from 2009 and projected to 2020, the billions of subscribership's can be modeled by C(t) = 0.000286t3 - 0.0443t2 + 1.49t - 5.36
Where t is equal to the number of years after 2000. When does this model estimate that the number of subscribers reached its maximum, and what maximum does it estimate? Source: portioresearch.com