Finding 2nd Vds value in a NMOS circuit
Both transistors conduct a current $$I_D=3.6mA$$. 

Assume both of them operate in saturation (constant-current operation); use the relevant equation to find the overdrive voltage.

Then use $$V_{OV}=V_{GS}-V_{t}$$ to find the source voltage of transistor 2. The rest is easy.