There are 60 minutes in an hour. Whatever distance a person crosses in 5 mins, he/she can cross 12 times that distance in 1 hour. So, in one hour the person could have crossed 12 X 600 = 7200 meters.
But, 1000 meters equal one kilometer. So, 7200m = 7.2 km. Thus, this person travels 7.2 km in one hour, and hence has a speed of 7.2 km/hr