Answer:
Explanation:
Given
mass of block 
at 
displacement is 
velocity 
acceleration 
suppose
is the general equation of SHM
where A=amplitude
=natural frequency of oscillation
therefore velocity and acceleration is given by


for t=0



divide 1 and 3 we get


Now square and 1 and 2 we get



The answer is 24N. Since the body is moving with constant velocity all the forces must balance (equal & opposite)