Razavi's Book Feedback Chapter Question

Hello guys =)

I would appreciate it very much if someone could give me a hand with this question from the 8th chapter (feedback) of the Razavi's book on analog design of CMOS circuits. My results don't match the solution given, which I don't understand how was obtained, so I need a more detailed solution to get it.

Here's the question:


And here's the solution that I don't get:


Thank you very much =)