The correct equilibrium expression would be one that has products over reactants, and to the power of their stoichiometric coefficient. Also, it would not include any liquids or solids.
The equilibrium expression for your balanced reaction is:
[tex]\Large K_c = \sf{\frac{[SO_3]^2_{equil}}{[SO_2]^2_{equil}[O_2]^2_{equil}}[/tex]
