Раскроем скобки и упростим:
a) $x - (x - (3x - 1)) = x - (x - 3x + 1) = x - x + 3x - 1 = 3x - 1$
б) $12y - ((x - y) + 12x) = 12y - (x - y + 12x) = 12y - x + y - 12x = 13y - 13x$
в) $5a - (6a - (7a - (8a - 9))) = 5a - (6a - (7a - 8a + 9)) = 5a - (6a - (-a + 9)) = 5a - (6a + a - 9) = 5a - (7a - 9) = 5a - 7a + 9 = -2a + 9$
г) $13b - (9b - ((c - b) - 9b)) = 13b - (9b - (c - b - 9b)) = 13b - (9b - c + b + 9b) = 13b - (19b - c) = 13b - 19b + c = -6b + c$