Решение:
- a) $$6m + 2n - 3m - 7n = (6m - 3m) + (2n - 7n) = 3m - 5n$$
- б) $$5x + (13 - 2x) - (6 + 12x) = 5x + 13 - 2x - 6 - 12x = (5x - 2x - 12x) + (13 - 6) = -9x + 7$$
- в) $$8 - 7(5b - 4) = 8 - 35b + 28 = -35b + 36$$
Ответ: a) $$3m-5n$$, б) $$-9x+7$$, в) $$-35b+36$$