The equilibrium expression for any chemical reaction can be determined if the phase of each substance that constitutes the chemical equation is known and the equation is balanced with the same number of atoms of each element on both the sides.
The equilibrium expression can be in terms of the concentration of each substance that makes up the equation or if there are gases involved it can be terms of the partial pressure. The equilibrium equation does not include substances that solids or pure liquids or solvents.
For the equation given: C6H12O6+ 6O2----->6H2O + 6CO2, C6H12O6 is being oxidized to give water and carbon dioxide.
C6H12O6 (s) + 6O2(g) -----> 6H2O(l) + 6CO2(g)
Here C6H12O6 is a solid and the water in the products is a liquid.
We can write the equilibrium constant
Kc = [H20]^6*[CO2]^6/[O2]^6, where [X] is the concentration of X in moles per liter and it is raised to power equal to the number of molecules in the balanced chemical equation.
Kp = [H2O]^6*P(CO2)^6/P(O2)^6 where P(X) is the partial pressure of X and it is raised to the power equal to the number of molecules in the balanced chemical equation.