Using the chemical reaction given above, we can rewrite the Equilibrium constant expression to reflect concentration. The concentration is molarity, written as moles per liter (M=mol/L).
The products are in the numerator and those of the reactants are in the denominator. The equation is a function of the concentrations, the upper case letters are the molar concentrations of the reactants and products (A, B etc.).
The lowercase letters are the stoichiometric coefficients that balance the equation.
Hence Answer is $K = [SO_3]^2/[SO_2]^2[O_2]$