This can be solved using the Le Chatelier's principle, which is widely used in order to predict the effect of the change of conditions on chemical equilibriums. According to Le Chatelier's principle if a chemical system at equilibrium undergoes a change in concentration, temperature, volume, or pressure, the equilibrium shifts in such a way so as to undo the effect of the change.
PCl5 (g) + heat↔ PCl3 (g)+ Cl2 (g)