What is the economically efficient level of emissions? Why?

Scientists have discovered that a novel pollutant called Schitt is being released into the air by two firms, Firm1 and Firm2, located in a small town called Rosebud. It has been determinedthrough rigorous medical and environmental researchthat the monetary value of damages caused by Schitt emissions are given by the marginal damage function MD = 4E. Note that E=(E₁+E₂) the aggregate emissions of the pollutant by Firm1 and Firm2. Firm1 has marginal abatement cost curve MAC₁ = 80 - 4E₁ and Firm2's marginal abatement cost curve for the pollutant is MAC₂ = 40 - 2E₂.