Acceleration = (change in velocity) / (time for the change)
Change in velocity = (ending velocity) - (starting velocity)
Change in the plane's velocity = (10,000 m/s north) - (8,000 m/s north)
Change in the plane's velocity = 2,000 m/s north
Time for the change = 40 seconds
Acceleration = (2,000 m/s north) / (40 seconds)
Acceleration = 50 m/s² north