Speed is measured in miles per hour (mph).
The formula Distance equals rate times time (d=rt) can be rearranged and the rate can be solved,
d = r*t => r = d / t . d = 100 miles, time = 6 hours, Rate = 16.66 mph (16 and two thirds).
The same formula can be used for the second part,
21000 / 24 = rate, Rate = 875 kilometers per hour