General Motors (GM) struggled from 2005 to 2012. In fact, GMs share of new-vehicle sales was 25.59%

Question:

General Motors (GM) struggled from 2005 to 2012. In fact, GMs share of new-vehicle sales was 25.59% in 2005, and it decreased by about 1.15 percentage points per year until 2012 (Source: GM). Let f(t) be GM’s share of new-vehicle sales at t years since 2005.
a. Find an equation of f.
b. Find f (7). What does it mean in this situation?
c. Find t when f (t) = 20.What does it mean in this situation?