HELP!! I need an explanation for this question!

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