In order to do this, you need to use the equation:
distance=rate x time
So if you want to find out how long it will take you plug 63 miles into the equation.
63 miles=Rate x Time
You want to find out how long so you need the rate.
You can find the rate from the first sentence.
21miles=Rate X 4 hours
Rate=21/4 miles per hour
Plug this into the original equation.
63=(21/4)(time)
Time=12 hours