Ответ: -17

[tex]9 - 7(x + 3) = 5 - 6x \\ 9 - 7x - 21 = 5 - 6x \\ - 7x - 12 = 5 - 6x \\ - 7x + 6x = 5 + 12 \\ - x = 17 \\ x = - 17[/tex]