Решим уравнение:
$$-x = 4 - 3(3x - 8)$$$$-x = 4 - 9x + 24$$$$-x = 28 - 9x$$$$-x + 9x = 28$$$$8x = 28$$$$x = \frac{28}{8}$$$$x = \frac{7}{2}$$$$x = 3,5$$
Ответ: 3,5