At a particular temperature 120 moles of so3 is placed

Question

Solve this particular problem:

At a particular temperature, 12.0 moles of SO₃ is placed into a 3.0L rigid containerand the SO₃ dissociates by the reaction

2SO_{3 (g)} <=> 2SO_{2 (g)} + O_{2 (g)}

At equilibrium, 3.0 moles of SO₂ is present.  Calculate K for this reaction.

It'd be nice to see how one would do this step-by-step and what I should be thinking when going through problems like these.  The subject for this problem would be 'Equilibrium Calculations'.