The forward and reverse reaction rates are different in the first 8 s, so the reaction is not at equilibrium in the first 8 s.
The forward and reverse reaction rates are identical after 8 s, so the reaction is at equilibrium after 8 s.
The upward sloping solid line represents the rate of the reverse reaction at any given time.
The rate of the reverse reaction would jump instantaneously to some higher value (say,
At the same time , the rate of the forward reaction would gradually increase to the same rate as the reverse reaction.