Answer
See below.
Work Step by Step
The coefficient $b$ is tied to the initial period, when the graph starts rising from the lowest values onward. During that initial period, the function is roughly exponential.
$b=1.09$ in exponential growth denotes $ 9\%$ rate of yearly growth.
Combining with the result of part (b), this exponential growth
of $ 9\%$ per year lasted until about $1979$ (11 years before 1990).
