We can solve this by converting each measurement to the same unit. I chose metres, but you could work with another unit if you are more comfortable with that.
6km. There are 1000 metres in one kilometre. Therefore 6 kilometres is 6000 metres.
10 miles. There are about 1609.34 metres in one mile. Therefore 10 miles is about 16093.4 metres.
7200 metres is already in metres.
With this information, we can tell that 6km is the shortest distance, followed by 7200m and 10mi.