the formula we use is
distance = speed * time
In the question we are provided with the info that
speed = 56 mph and
time = 3 hours 45 minutes = 3 + 45/60 hours = 3+ 0.75 hours = 3.75 hours
Now lets plug the values in the formula
distance = speed * time = 56 mph * 3.75 hours
distance = 210 miles
