The term "fast fashion" refers to low-quality clothing items produced cheaply in mass quantities. As a buyer typically only wears each fast fashion piece a few times, the fast fashion industry creates extra factory pollution and landfill waste. The market price of a fast-fashion T-shirt is $12, but the marginal external cost is $2.50 per shirt. What is the marginal social cost of a fast-fashion T-shirt at the market equilibrium quantity?