Thank you for the opportunity to help you with your question!

Kc=[CO2]

Normally you multiply the concentrations of the products (right side of equation) in the numerator and divide by the multiplication of the concentrations of the reactants (left side of equation) (in this case there is only one) but you do not include reactants or products that are in a solid state. They have the (s) after their chemical formula. This is why CO2 is the only term in Kc because its is a gas (g). You also raise each concentration to the power of the coefficient in front of the chemical formula in the equation. In this case all the coefficients are 1 so it is not necessary to write because anything raised to the power of 1 is itself.

Please let me know if you need any clarification. I'm always happy to answer your questions.