See that it is an isobaric expansion .
so , dQ = nCp dT
again dQ/dt = KA ( Ts - T ) /X ;
combining these two eqns , we get
dT/ ( Ts - T ) ={ KA/( n Cp X ) } dt
integrating ( taking final temp as Tf )
Ts - Tf / ( Ts - T ) = exp ( KA t / X n Cp )
from this reln we find Tf
Then from PVf = n RTf
we find Vf
again from PV i = n RT ( as given )
we find Vi
Now we find increase in volume ( Vf - Vi )
distance moved = ( Vf - Vi ) / A