Imagine a market where there is a negative consumption externality. The private inverse demand function for the good is PD,p = 11 - 2QD,p. The social inverse demand function that accounts for the negative externality is PD,s = 8 - 2QD,p. The (social and private) inverse supply function is PS = 2 + QS. What is the social optimum (as opposed to the market equilibrium) quantity for the good?