What is the expression for the equilibrium constant, Kc, for the following chemical reaction

PbO(s) + CO(g) = Pb(l) + CO2(g)

Kc will always equal products / reactants. However, you never ever include products/reactants that are in pure solid or liquid form. Therefore, the Kc for this reaction is [CO2] / [CO]

Actually, liquid is included to get the equilibrium constant. The term that you can never include in is only for solids. So the constant can be written as
Kc = [CO2][Pb] / [CO]