Small Signal MOSFET Transconductance Gain - Can't Figure it Out

[willbanks]

I am tasked to find the gain of this small signal circuit drawn in my work. I have absolutely, NO idea how the professor came up with his answer. I would e-mail him, but he doesn't respond on weekends.

The professor's answer is included. If anyone can tell me where I am messing up, I would be incredibly grateful. the channel length modulation (lamda) = 0, so the output impedance is infinite therefore an open circuit.



https://imgur.com/a/tQGSZr8