Replace the MOSFET with its small signal model. So you have a current source from drain to source, whose value depends on the transconductance as well as the gate-to-source voltage. You don't have body effect in this case, right? Then, consider the output resistance due to channel length modulation. Once you put these together, you basically have one node where you can use KCL - you've reduced the circuit to a very simple linear circuit.
Also consider what happens to VDD in the SSM - AC ground.
Show us what you have and where you're stuck.