because the Kp expression is
Kp= p(products) / p(reactants)
so the Kp expression for this equilibrium wil always be
Kp= (p(Cl2) x p(PCl3)) / p(PCl5)
we've been given the partial pressure for Cl2, which is the mole ratio x total pressure. total pressure will be constant, and the mole ratio you can expect to be the same as PCl3 because if we did an ICE table, we'd find that both values started at 0 and went up by the same amounts, as they have a 1:1 molar ratio. so therefore we can assume that p(Cl2)=p(PCl3) for this reaction.
therefore
Kp= p(Cl2)^2/p(PCl5). plug your numbers in and there ya go!