Hello. First one will need to balance the equation, because the coefficients are important when solving for Kc.
If you agree with a balanced equation of 2C12H26 + 37O2 <----> 24CO2 + 26H2O, then we may proceed.
To solve for Kc, we put products over reactants (products divided by reactants) as in the following equation:
Kc = [1.1 x 103]24[1.1 x 103]26 / [1.8 x 103]2[8.6 x 103]37
Hopefully that helps...