Hi there!
In order to find your answer, you'll need to compare for the same time how many miles can both run :
35 miles → 1 hour
21 miles → half hour = 42 miles → 1 hour
Now you can easily see that a fox runs faster than a rabbit, because a fox can run 7 more miles per hour than a rabbit.
There you go! I really hope this helped, if there's anything just let me know! :)