The expression is .

                   (Remove parentheses)

Apply commutative property of addition: a + b = b + a.

Apply assosiative property of addition: a + (b + c) = (a + b) + c.

Combine like terms using distributive property: .

                  (Apply additive inverse property: )

                       (Apply zero product property: )

                                (Add: 3 + 3 = 6)

                                      (Apply additive identity property: 6 + 0 = 6)

The simplified expression is 6.