Set the downwind flight time x hours and the headwind time y hours.

x+y=8

900x=700y

Solution:

x=3.5

y=4.5

Then the flight distance s = 900 * x = 900 * 3.5 = 3 150km.

If you don't know this equation:

The flying distance is 900 * 8 * (700/(700+900) = 3150km.

(It can be understood that the distance between the downwind and the headwind must be the same, so the flight time should be inversely proportional to the speed, that is, 700/(700+900))