Решение:
6 = 8 - 5 * 7x - 5 * (-1)
6 = 8 - 35x + 5
6 = 13 - 35x
35x = 13 - 6
35x = 7
x = 7 / 35
x = 1 / 5
x = 0.2
Ответ: 0.2