Equilibrium shifts and Kc
Kc is products over reactants with the coefficients as exponents, and a stressed system shifts so as to partly undo the stress.
A worked example
For N2 + 3H2 in equilibrium with 2NH3, with [N2] = 0.50, [H2] = 0.20 and [NH3] = 0.40 mol/L, Kc = 0.40^2 / (0.50 x 0.20^3) = 40.
The mistake people actually make
A catalyst reaches equilibrium sooner but never moves its position. Treat heat as a product of an exothermic reaction, so cooling drives it towards the products.