Step-by-step explanation:
Given that,
Mass of bus, m = 6110 kg
Speed of bus, v = 20 m/s
The bus will stop in 24 s gently applying the brakes, t = 24 s
The average force exerted on the bus is given by :

If the driver slams on the brakes, the stops in 3.90 s, t' = 3.9 s
The average force exerted on the bus at this time is given by :

Hence, this is the required solution.