Hello, I was curious if you could tell me where they got the equation for Vgs = (Vth + Id/gm) on number 1 for finding Rd4, and if this equation is...

Hello, I was curious if you could tell me where they got the equation for Vgs = (Vth + Id/gm) on number 1 for finding Rd4, and if this equation is correct at all times or only in certain cases. Also, what allows them to assume the velocity limited model and use the equation for gm = Vsat*Cox*Wg? They don't give gate lengths or mobility so I'm confused on that. Thanks!