quadratic function problem where no equation is given

This is the question I am supposed to answer...

A jet flew from Tokyo to Bangkok, a distance of 4800km. On the return trip, the speed was decreased by 200km/h. If the difference in the times of the flights was 2 hours, what was the jet's speed from Bangkok to Tokyo?

I have to create an equation that represents this problem and then use it to solve the problem. The only thing I can think of so far is that since speed will decrease by 200km at a time, so I will let x represent the number of 200km increments.

Other than that I am totally lost so anyone that knows anything would be a big help!! thank you :)