We know workdone by the gas is given by,
⇒W=∫PdV
It is given, PV=k (constant).
So, W=∫V2V1kV dV =k2[V22−V21]....(i)
As, ⇒PV=k⇒nRTV2=k⇒T2T1=(V2V1)2⇒(V2V1)2=15.....(ii)
From (i) and (ii),
W=k2[15V21−V21] =142kV21=7kV21....(iii)
We know, PV=nRT⇒kV21=1×RT⇒kV21=RT....(iv)
From (iii) and (iv),
⇒W=7RT, Hence x=7.