6x - (7x - (8x - (9x + 1))) = 6x - (7x - (8x - 9x - 1)) = 6x - (7x - (- x - 1)) =
= 6x - (7x + x + 1) = 6x - (8x + 1) = 6x - 8x - 1 = - 2x - 1
6x - (7x - (8x - (9x + 1))) = 6x - (7x - (8x - 9x - 1)) = 6x - (7x - (- x - 1)) =
= 6x - (7x + x + 1) = 6x - (8x + 1) = 6x - 8x - 1 = - 2x - 1