At a particular temperature, 12.0 moles of SO₃ is placed into a 3.0-L rigid container, and the SO₃ dissociates by the reaction 2SO₃ 1 g2m2SO₂ 1g2 1 O₂ 1g2

At equilibrium, 3.0 moles of SO₂ is present. Calculate K for this reaction. (The answer is 0.056) I just do not understand how they got that answer.