Finding Efficiency from a P-V Graph

[yossup]

uh...just wondering but how do you find out the efficiency from a P-V graph?

 

[Gear300]

depends on the engine's process

 

[yossup]

The figure is a PV diagram for a reversible heat engine in which 1.0 mol of argon, a nearly ideal monatomic gas, is initially at STP (point a). Points b and c are on an isotherm at T = 423 K. Process ab is at constant volume, process ac at constant pressure.

How would I go about finding the efficiency from such a graph?