Spring with two objects

[hannam]

Homework Statement

Object A (1 kg) and B (3 kg) are forced together,
initially compressing a spring S between them by 6 cm;
then the system is released from rest. The spring with
k=1250 N/m is not fastened to either block and drops to
the surface after it has expanded to its unstretched position.
a) By assuming frictionless surface, what are the speeds of objects after the spring has expanded
to its unstretched position?
b) Assume that the surface is frictional with s=k=0.1. If object B has speed of 0.2 m/s at 0.5 s
after the system is released from rest, what should the speed of object A be at this time instant?
(Hint: Consider “center of mass of a system” concep

Homework Equations

W=1/2kx^2
F=k.x
K=1/2mV^2

The Attempt at a Solution

i'm having trouble with this energy thing. i tried 1/2kx^2=1/2mv1^2+1/2mv2^2
i found 37,5=1/2v1^2 + 1/2v2^2
and i tried F=k.x=75 .
then used f=m.a formula both objects and found V1=3V2. so i solved equation but it seems all wrong.
can you help?

 

[hannam]

and i have no idea what to do in part b :)

 

[hannam]

can anyone help ? :(