Answer:
Tax = 120 per unit
Explanation:
First lets calculate Socially optimum Marginal Profit,
This can be computed by factoring in the marginal external benefit in the costs.
thus,
PMB = (PMC + MD)
This gives us the socially profit maximizing level of Product Q in quantity produced terms. we solve for Q the above equation. As generally profit is maximized at Marginal cost = Marginal Revenue
540 - 6Q = 8Q + 4Q
540 = 18Q
Q = 30 units
Since the tax amount should equal the value of marginal external damage so the amount of tax at socially optimum level thus is,
MD = T,
that gives T = 4 * 30 = 120 per unit
This shall then give the revised Marginal cost function of
Revised marginal cost = 8Q + 120. This corrects the externality by making production more expensive.
Hope that helps.