Verified answer

(x² + 12x)² + 18(x² + 12x) + 81 = 0

Сделаем замену :

x² + 12x = m    

m² + 18m + 81 = 0

(m + 9)² = 0

m + 9 = 0

m = - 9

x² + 12x = - 9

x² + 12x + 9 = 0

D = 12² - 4 * 9 = 144 - 36 = 108 = (6√3)²