Answer:
See below
Step-by-step explanation:
Divide the distance he walks by the speed that he walks.
21km
-------- = 3 s
(7km/s)
When you divide the top number by a number with km/s, you would flip that ratio over.
So your units would be
km x s/km the km cancel each other out and you are left with s.
You just divide the numbers as the are. 21 / 3 = 7