Question

At a particular temperature, 13.4 mol of SO3 is placed into a 3.8-L rigid container, and the SO3 dissociates by the reaction given below.

2 SO3(g) <-> 2 SO2(g) + O2(g)

At equilibrium, 3.6 mol of SO2 is present. Calculate K for this reaction.

Answer 1

I assume you want Kc and not Kp.
13.4 mol/3.8 = approx 3 but that's just an estimate. You can recalculate and use a more accurate number.

........2SO3 ==> 2SO2 + O2
I.......3.........0.....0
C......-2x........2x....x
E......3-2x.......2x....x

You are given mols SO2, convert that to M (which is x), evaluate 2x and 3-2x. plug those numbers into Kc expression and solve for Kc.
Post your work if you get stuck.