Ok, you have already figured out the speed of the jet going both directions and I will trust that you have done those correctly.
With the jetstream = 940 km/h
Against the jetstream = 700 km/h
Let's call the speed of the jet and the speed of the jetstream .
When traveling with the jetstream:
When traveling against the jetstream:
We know that the absolute speed of the jet is the same in both cases, so let's isolate in the first equation:
Now sub that into the second equation:
Solve for y, then sub that value into either of the first two equations to solve for x.