In 2010, the total number of overnight visits to national parks was 13.91 million visits, and it

Question:

In 2010, the total number of overnight visits to national parks was 13.91 million visits, and it decreased by about 0.21 million visits per year until 2014.
a. Find an equation of a model to describe the situation. Explain what your variables represent.
b. Estimate in which year there were 13.0 million visits.