Use the work-energy theorem. The total work done on the plane is equal to the change in its kinetic energy:
W = ∆K
W = 1/2 (1.5 kg) (6 m/s)² - 1/2 (1.6 kg) (8 m/s)²
W = -21 J
(Note that the work must be negative since the "braking" force opposes the direction in which the plane is moving.)
