Verified answer

Пусть x^2 - 6x = t, тогда t (t - 21) + 80 = 0  

t^2 - 21t + 80 = 0 

D = 121

t1 = (21 + 11)/2 = 16

t2 = (21 - 11)/2 = 5

Имеем 2 случая

1) x^2 - 6x = 16

2) x^2 - 6x = 5

Решим первое

x^2 - 6x - 16 = 0

x^2 - 6x + 9 - 25 = 0

(x - 3)^2 - 25 = 0

(x - 3 - 5)(x - 3 + 5) = 0

(x - 8)(x + 2) = 0

x1 = 8

x2 = - 2

Решим второе

x^2 - 6x - 5 = 0

x^2 - 6x + 9 - 14 = 0

(x - 3)^2 = 14

x - 3 = ±√14

x3 = 3 + √14

x4 = 3 - √14