Answer:
2.5 s
Step-by-step explanation:
We use the equation of motion for rotational motion:

where
is angular displacement,
is initial angular velocity
is the angular acceleration
is the time
Substituting values,


Before the 9.0 s interval, this value is the final angular velocity. The initial angular velocity is 0 rad/s since it started from rest.
We use the equation:

Note that
is the value we got earlier and
is the initial angular velocity and is equal to 0 rad/s.

