Решение:

1. а) $$5a + 27a - a = (5 + 27 - 1)a = 31a$$
2. б) $$12b - 17b - b = (12 - 17 - 1)b = -6b$$
3. в) $$6x - 14 - 13x + 26 = (6 - 13)x + (26 - 14) = -7x + 12$$
4. г) $$-8 - y + 17 - 10y = (-1 - 10)y + (17 - 8) = -11y + 9$$