Since neither of the bracketed expressions is preceded by a minus sign, and neither one has a multiplier outside of it, all of the parentheses can simply be removed.
(3x - 5) + (3x + 6)
3x - 5 + 3x + 6
Combine the 'x'-terms: 3x + 3x = 6x
Combine the numbers: -5 + 6 = 1
The expression is equivalent to . . . . . 6x + 1