Answer:
Alexander would enhance his stereo playing for 100 dollars more. Mary would pay a lawyer 100$
Step-by-step explanation:
Socially optimal solution is the situation where all external costs are taken into account, as well as internal costs and benefits. Therefore, If Alexander's benefits from playing the stereo is 250 dollars and Mary's costs are 350 dollars, than in order to get socially optimal solutions, Alexander could enhance his stereo playing for 100 dollars more. Mary would pay a lawyer up to 100 dollars.