We have the distance: d = 400 km and we have the time: t = 4.5 hours.
The formula for the speed ( the velocity ) is:
v = d / t ( distance/time )
So v = 400 km / 4.5 h
v = 88.89 km/h
The SI unit is m/s.
If we want to transform km/h to m/s we have to divide this result by 3.6:
88.89 km/h : 3.6 = 24.69 m/s
Answer: The speed of the bus is 88.89 km/h or 24.69 m/s.