a = -2/3 => (a - 2b) + 4b(a - b) = (-2/3 - 2b) + 4b(-2/3 - b) = -2/3 - 2b - 8/3 * b - 4b² = -4b² + (-2 - 8/3)b - 2/3 = -4b² - 14/3 * b - 2/3