Решение:
(х + 3)² = 3x² + 6x-7
x² + 6x + 9 = 3x² + 6x - 7
x² - 3x² + 6x - 6x + 9 + 7 = 0
-2x² + 16 = 0
-2x² = -16
x² = 8
x = \$$\$$\pm\sqrt{8}\\\$$\$$
x = \$$\$$\pm 2\sqrt{2}\\\$$\$$
Ответ: \$$\$$\pm 2\sqrt{2}\\\$$\$$