Signal flow graph for two-port network

Hi, this is part of an application note entitled "Low-Noise Amplifier Stability Concept to Practical Considerations" from Maxim.
Could you help me with the question in the picture? Thanks.
Source: http://www.maximintegrated.com/app-n...ex.mvp/id/1849

Hi
The solution could be find here. The rules which are used are explained here.

Thank you, Selehkhan. Great lecture ever. I am reading lectures from that link and will come back later with that problem.