(b) $C_{P} - C_{V} = \frac{R}{J} \Rightarrow C_{P} = \frac{R}{J} + C_{V} = \frac{R}{J} + \frac{R}{J(\gamma - 1)}$
$\Rightarrow C_{P} = \frac{R}{J}\left( \frac{\gamma}{\gamma - 1} \right) = \frac{R}{J}\left( \frac{1.5}{1.5 - 1} \right) = \frac{3R}{J}$