Answer
$t\approx-9.5$
In 1981, the number of teams was rising at the fastest rate (per year).
Work Step by Step
From the graph, we see that the curve is steepest at around $t=-8.06$.
Thus: $t\approx-8.06$
Interpretation:
8.06 years BEFORE 1990 (sometime in 1981), the number of teams was rising at the fastest rate (per year).
