Explanation:
I am not sure I fully understand what your teacher wants from you, but I would say we only need to define the burning rate per hour for both people, and with that normed rate (number / hour) we can answer the main question directly.
Thomas burned 675 cal/ 2 hours.
by dividing to and bottom by 2 (it is important to do the same action on the top and the bottom to keep the value of the ratio unchanged) we get
337.5 cal / hour
and Marvin burned 1035 cal / 3 hours.
so, the same trick, this time dividing to and bottom by 3 to norm the whole ratio to a 1 hour baseline :
345 cal / hour
so, Marvin burned more calories per hour.