Doubt from thermodynamics

How to do q47 using WPE method..we have been told to use this type of procedure...

I am not getting any value of H from that equation.. where is the mistake
help @Shwetanshu_2018

I did by using simple force equation balance.

Given massless piston so net force on pistons is zero at any instant.Take initial and final instants .
x in the spring force represents compression in it which is (h/16)
P(initial)=1 atm(NTP)


Thanks !! I understood from your solution how to do it by the force method... But please can you see where I was wrong in that method of doing by WE therom @Bhuvanitha_2020