you're probably just forgetting to divide by liters to get molarity. Any K equilibrium constant is defined as the [products]/[reactants], thus ((0.4*14.25)/3) is your molarity of each product, 40% of 14.25 moles divided by the volume in liters to get molarity, and your concentration of reactant is (14.25*.6)/3, thus gives ((1.9)^2)/2.85=k I just wrote it as squared but it really means that each of your products concentration is 1.9M because they decomposed from the same reactant.
