P-V diagram of an ideal gas is shown in Fig. Work done by the gas in the process ABCD is :

Work Done in process AB is 

${\mathrm{W}}_{\mathrm{AB}}={\mathrm{P}}_0\left({ \mathrm{V}}_0-2{ \mathrm{V}}_0\right)=-{\mathrm{P}}_0{ \mathrm{V}}_0$

Work done in path BC is zero since volume remains constant.

Work done in path CD is :

${\mathrm{W}}_{\mathrm{CD}}=2{\mathrm{P}}_0\left(3{ \mathrm{V}}_0-{\mathrm{V}}_0\right)=4{\mathrm{P}}_0{ \mathrm{V}}_0$

$\text{ Net work done in path }\mathrm{ABCD}\text{ is }\mathrm{W}={\mathrm{W}}_{\mathrm{AB}}+{\mathrm{W}}_{\mathrm{CD}}=3{\mathrm{P}}_0{ \mathrm{V}}_0$