please help me with this question. I am very confused.

The inverse demand function for a good is P = 200 2Q, and the inverse supply curve is 20 + Q. Production of the good creates a positive externality of 15 per unit produced. Left alone, how much will the market produce? What is the socially optimal amount of production? A subsidy of how much per unit could get this market to the socially optimal outcome? (Do not include a 33 sign.)