In a vessel containing SO3,  SO2  and O2 at equilibrium, some helium gas is introduced so that the total pressure increases while temperature and volume remains constant. According to Le-Chatelier principle, the dissociation of SO3

2SO3 ⇌ 2SO2 + O2∆H=+ve  endothermic reactionInert gas doesn't show reaction with the reacting mixture it shows effect only due to its volume.At constant volume the change doesn't effect the inert gas. Thus dissociation of SO3 remains unchanged.V = constant = ∆H=0No effect on the inert gas.