Final answer:
The imposition of a $6 per unit tax in a market generally results in a new equilibrium with a lower quantity due to the upward shift in the supply curve. The specific effect on the equilibrium quantity cannot be determined without further information on market demand and supply curves.
Step-by-step explanation:
The question pertains to the effect of a tax imposition on the equilibrium quantity in a given market. In general, the imposition of a tax leads to an increase in the cost of producing or selling each unit of the good, which typically results in the supply curve shifting upwards or to the left. Consequently, at the original equilibrium price, the quantity supplied decreases while the quantity demanded remains the same, resulting in a new equilibrium with a lower quantity and higher price than before the tax was imposed.
The specific equilibrium change due to the tax cannot be determined without additional information on the demand and supply elasticities or the pre-tax supply and demand curves. However, from the reference provided, after considering the external costs, the equilibrium price would be $12 and the equilibrium quantity would be four units. Applying this logic, after imposing a $6 per unit tax, we would expect a similar outcome of reduced equilibrium quantity, though the exact figures would depend on the aforementioned demand and supply sensitivities.