A pharmaceutical corporation has two plants that produce the same over-the-counter medicine. If x₁ and x₂ are the numbers of units produced at plant 1 and plant 2, respectively, then the total revenue for the product is given by R = 200x₁ + 200x₂ - 4x²₁ - 8x₁x₂ - 4x²₂. When
x₁ = 4 and x₂ = 12, find 

(a) The marginal revenue for plant 1,
∂R/∂x₁

(b) The marginal revenue for plant 2, ∂R/∂x₂