All surfaces shown in fig are smooth .System is released with the spring unstretched.In equilbrium ,compression in the spring will be:

( nothing is fixed in the ques except spring which is fixed from wall)

Plz do it with energy conservation coz i am not able to do it with energy conservation 

answer is (mg/k)

this answer is 100% correct u can check it by solving this ques from newton's laws

fig is below 

plz click on fig & open it then u will see clear picture here it is not clear