n² + n¹¹ - n³ - n² - n³ + n¹² = (1 - 1)n² + n¹¹ + (-1 - 1)n³ + n¹² = n¹¹ - 2n³ + n¹²
Ответ: n¹¹ - 2n³ + n¹²