initial partial pressure SO3 .500atm at 1000k. At equilibrium SO3 pressure is .200 atm at 1000k. Kp at 1000k?

Question

This is in a sealed container. 2 so3 is in equilibrium with 2 so2 and o2 (all gases) . 2 SO3(g)<~~~~>2 SO2(g) + O2(g) .The chemistry book say the answer is 0.338. For some reason i am getting either .100 or 6.35. Please help. The textbook is Chemistry central science by brown lemay bursten. 10th edition. Chapter15.5 practice exercise on pg. 645. Thank You.

Answer 1

2 SO3 <------> 2 SO2 + O2
initial pressure
0.500
at equilibrium
0.200 . . . . . .. 0.300 . . .0.150
Kp = (0.300)^2 x 0.150 / 0.200 = 0.0675