Hi thank you! But i still dont really get why -
a=3

Tangent graph max and min goes to infinity so there is not really an “amplitude” for tangent graphs. The amplitude for tangent graph is actually telling us how steep/shallow the graph is and the points we use to determine is the 1/4 or 3/4 points of the graph / 1/4 or 3/4 the length of the left to right asymptotes.

Whether a is positive or negative depends on whether the slope is in the same direction as the standard tangent graph (bottom left to top right).

The standard tangent graph y = tanθ,
|a|=tan(-π/4) or tan(π/4) = 1. So for your question a=3 means the graph is steeper than the standard tangent graph and in the same direction.