Airspeed is speed with respect to the air, i.e. speed through the air.

Groundspeed is speed with respect to the ground, i.e. speed over the ground.

It's the vector sum of airspeed and windspeed.

900 kmp

The Dreamliner´s groundspeed at cruising altitude is about 480kts. / 550mph. / 890 km/h.

u said airspeed remains the same, so 840 mi in 3 hrs the groundspeed is 280, for the return trip groundspeed is 240, therfore the wind speed is 40

Air speed is the speed through the air (surprise!) and the air is often moving, you can have a strong tailwind or headwind so that will affect your speed over the ground (groundspeed). The speed through the air might be 500mph, and the groundspeed either 550mph or 450mph, if the wind is at 50mph.