Since the fox's rate is based on half an hour and the rabbit's rate is based on a whole hour, you would need to create an equation that is spanned on the same amount of time.
To do this you would, simply, multiply the fox's rate by 2.
21 x 2 = 42

x 2 = 1
42 mi/1 hA fox can run 7 miles per hour faster than a rabbit can. I hope this helps!