You can write it as:
Rabbit = 35mi in 1 hour
Fox = 21mi in 0.5 hours
To change the 0.5 hours into 1 hour, we need to multiply it by 2 so...
21*2 = 42
Then we can calculate the difference and write the answer:
42-35 = 7
The Fox runs faster than the rabbit by 7 miles an hour.
Hope this helps! :)