Kp = 9.3x10⁴
Explanation:
The reaction is this:
4PBr₃ (g) ⇄ P₄ (g) + 6Br₂ (g)
We define Kp from the partial pressures in equilibrium
PBr₃ → 97.4 atm
P₄ → 99.2 atm
Br₂ → 97.2 atm
Kp = (Partial pressure Br₂)⁶ . (Partial pressure P₄) / (Partial pressure PBr₃)⁴
Kp = 97.2⁶ . 99.2 / 97.4⁴
Kp = 929551.4533
929551.4533 → 9.3x10⁴