Kevin is collecting data about the number of people that use smartphones in a certain region of the world. the number of people using smartphones in that region went from 64.8 million in 2010 to 278.6 million in 2018, and can be modeled by the function f(x)=64.8(1.2)x , where x is the number of years since 2010, and f(x) is the number of people that use smartphones, in millions.