Assuming a constant force, and hence a constant acceleration a, we have by Newton's second law,
4.0 N = (1.2 kg) a → a = (4.0 N) / (1.2 kg)
and by definition of average velocity,
a = (5.0 m/s - 2.0 m/s) / t
where t denotes the time for which the force is applied.
So,
(4.0 N) / (1.2 kg) = (3.0 m/s) / t → t = (3.0 m/s) (1.2 kg) / (4.0 N) = 0.90 s
