Answer
$t\approx-10.3$
In 1979, the number of teams was rising at the fastest rate (per year).
Work Step by Step
Zooming out of the graph to a window in which we see negative values of t, we see that the slope stops rising at about $t\approx-10.3$
and stagnates from that point forward.
Interpretation:
10.3 years BEFORE 1990, the number of teams was rising at the fastest rate (per year).
Thus: $t\approx-10.3$
Hence: In 1979, the number of teams was rising at the fastest rate (per year).
