the reaction of two gases in a sealed container produces a different gas and the system reaches equilibrium. what change in concentration would shift the equilibrium to the left?

Increasing the concentration of a reactant or decreasing the concentration of the product.

Explanation: For the equilibrium of the reaction to shift to the left, we must decrease the concentration of a reactant or increase the concentration of the product. This is based on Le Chatelier's principle which describes the effect on the equilibrium of changes in concentration or pressure of a product or a reactant. Adding more products in a system would disturb the equilibrium thus it would cause to reestablish it by shifting the reaction to the left producing the reactants. This would also be the same when you decrease the number of reactants in the system.