x and y are real numbers such that x² + 3y = 9, and 2y² - 6x = 18. If the 4 solutions to the system are (x₁,y₁), (x₂,y₂), (x₃,y₃), and (x₄,y₄), then find (y₁ - x₁) + (y₂ - x₂) + (y₃ - x₃) + (y₄ - x₄).