One jet flies at a ground speed of 75 miles per hour faster than another jet. on a particular flight the faster jet requires 3 hours and the slower one 3 hours and 36 minutes. what is the length of the flight

It is a distance-speed-time problem. Applying the general formula of , we'll solve this problem. If the speed of the second jet is , then the speed of the first jet is  according to the problem. The first jet requires 3 hours and 36 minutes (3.6 hours) and the second one 3 hours. 
By setting up an equation we can write that  and 
The length of flight is or miles