Answer
$\Delta S=-0.0384\ kJ/K$
Work Step by Step
From polytropic processes:
$\dfrac{T_2}{T_1}=\left(\dfrac{v_1}{v_2}\right)^{n-1}$
Given $T_1=310\ K,\ n=1.3,\ \dfrac{v_1}{v_2} =2$
$T_2=381.7\ K$
$\Delta S = m\left[c_v\ln\left(\dfrac{T_2}{T_1}\right)+R\ln\left(\dfrac{v_2}{v_1}\right)\right]$
With $m=0.75\ kg,\ c_v=0.743\ kJ/kg.K,\ R=0.2968\ kJ/kg.K$
$\Delta S=-0.0384\ kJ/K$
