We know, 3 feet = 1 yard.
We have to convert feet to yards to calculate the ratio so that the final answer doesn't have any unit.
Therefore, 21 feet = 
 = 7 yards
 = 7 yards
Thus, the ratio 

The fraction 
 is already in its simplest form.
 is already in its simplest form.
Therefore, the given ratio as a fraction in the simplest form without any units is 
