Bonjour
x² + 6x + 9 = 2(x + 3)(x + 7)
x² + 2(x)(3) + 3² = 2(x + 3)(x + 7)
(x + 3)² = 2(x + 3)(x + 7)
(x + 3)² - 2(x + 3)(x + 7) = 0
(x + 3) [(x + 3) - 2(x + 7)] = 0
(x + 3)(x + 3 - 2x - 14) = 0
(x + 3)(-x - 11) = 0
x + 3 = 0 ou -x - 11 = 0
x = -3 ou x = -11
S = { -11 ; -3 }