Given data:
The initial numbers of follower areA=500,000.
The rate at which followers increases is r=19% =0.19.
(a)
The expression for the exponential growth model is,
![F=A(1+r)^t](https://img.qammunity.org/2023/formulas/mathematics/college/3w9qev8pirlzt817u3kqcw79ecv163fmzl.png)
Substitute the given values in the above expression.
![\begin{gathered} F=(500,000)(1+0.19)^t \\ =500,000(1.19)^t \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/4yoha7hb8fyrhcoeupoe2qwveqwqwb4g0e.png)
(b)
The value of time is 7 in the year 2019, substitute 7 for t in the above expression.
![\begin{gathered} F=500,000(1.19)^7 \\ =1689,657.7 \\ \approx1690,000 \\ \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/vmajhvwc7wb7v3g4jpf7a4o0yo9yj1ydzs.png)
Thus, the expression for the exponential model is f=500,000(1.19)^t, and the followers in the year 2019 is 1690,000.