C + CO2 --> 2 CO
First, I would recommend including the phase subscripts for each molecule in the reaction. When calculating the equilibrium constants, solid and liquid concentrations are constant, so they are omitted from the equilibrium equation.
Next, you will need to find the molarity of each molecule involved:
M = mols solute / L solvent
K = concentration of products / concentration of reactants or:
K = [CO]^2 / [CO2] (where [] are Molarities... and I am assuming carbon is solid and the others are gases)
Hope that helps a little