Assume that you have 2 L of an ideal gas that is at atmospheric pressure (1 atm).

a. The gas is cooled at constant pressure to 1/2 its original volume.

b. It is then expanded isothermally back to 2.0 L.

c. At that point the pressure is increased at constant volume until the original pressure is reached.

Draw the PV diagram

a. For the compression at constant pressure (result of cooling), you have a horizontal line from 2.0 L to 1.0 L (P vertical, H horizontal axis as is customary)

b. As it is expanded isothermally back to 2.0 L, the curve follows a P = constant/V shape

c. The volume stays constant and so the path back to original pressure is a straight vertical line