Efficient thermodynamics.

Two moles of a monoatomic ideal gas $\left( \gamma = {5}/{3} \right)$ is taken through a cyclic process $ABCA$ as shown above. The process $CA$ is best described as $PT = \text{constant}$ . If the efficiency of the cyclic process is

$\large \eta = 1 - \dfrac{k}{4 \ln 2 + 5}$

then what is the value of $k$ ?