The equilibrium constant for a given reaction
aA + bB -><- cC + dD is given as
Kp = (C)^c x (D)^d / ((A)^a x (B)^b)
where (C) is the partial pressure for gas C.
In the given case, Kp = (SO3)^2 / ((SO2)^2 x (O2))
You will need to substitute the values of partial pressure for each of these gases to get the answer.