б) $$(6x - 8) - 5x - (4-9x) = 6x - 8 - 5x - 4 + 9x = (6x - 5x + 9x) + (-8 - 4) = (6 - 5 + 9)x - 12 = 10x - 12$$
Ответ: $$10x - 12$$