Convert first the unit of their distances and their time.
Since 1 mile = 1.60934 km, then
300 miles * (1.60934 km / 1 mile) = 482.802 km
Since 1 hour = 60 minutes, then
5.5 hrs * (60 min / 1 hr) = 330 minutes
The speed is distance over time.
482.802 km / 330 minutes = 1.463 km/hr
Hope this helps ☺️ ✌️