Concentrations of gases at equilibrium
In cases like this calculate the reaction quotient, Q (https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Equilibria/Chemical_Equilibria/The_Reaction_Quotient) using the initial concentrations, and compare the value with the equilibrium constant, K:


Q=K implies that the system is at equilibrium
Q > K means that there is an excess of products so the system will evolve towards the equilibrium consuming products (which means that you should subtract x from the products)
Q &lt; K just the other way around, the system will shift to the right to reach equilibrium (subtract x from the reactants)