(3a + b)² + (3ab - 1)² - (3a - b)² =
= (9a² + 6ab + b²) + (9a²b² - 6ab + 1) - (9a² - 6ab + b²) =
= 9a²b² + 6ab + 1 = (3ab + 1)²
Ответ: (3ab + 1)²