Box moving with velocity 3m/s stops after travelling distance of 12m.Retarding force acting on it ?

Answer 1

with uniform acceleration (deceleration) to a stop
... the average velocity is half of the initial velocity

12 m / (1.5 m/s^2) = 8 s

acceleration = (3 m/s) / 8 s = .375 m/s^2

the retarding force is proportional to the mass of the box
... f = m a
... need the mass to find the force