$\Rightarrow \Delta \mathrm{U}=-6 \mathrm{R} \quad(\because \mathrm{W}=6 \mathrm{R})$
Therefore $-6 R=1\left(\frac{R}{\gamma-1}\right) \Delta T=\frac{3}{2} R \Delta T$
$\Rightarrow \Delta \mathrm{T}=-4 \Rightarrow \mathrm{T}_{\mathrm{final}}=(\mathrm{T}-4) \mathrm{K}$
