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