а) $$ x + 3 + (6x - 8) = x + 3 + 6x - 8 = 7x - 5 $$
б) $$ (5x - 2) - (8x - 5) = 5x - 2 - 8x + 5 = -3x + 3 $$
в) $$ 30 + 5(0.2y - 6) = 30 + y - 30 = y $$