15 m/s^2
The first thing to calculate is the difference between the final and initial velocities. So
180 m/s - 120 m/s = 60 m/s
So the plane changed velocity by a total of 60 m/s. Now divide that change in velocity by the amount of time taken to cause that change in velocity, giving
60 m/s / 4.0 s = 15.0 m/s^2
Since you only have 2 significaant figures, round the result to 2 significant figures giving 15 m/s^2
