He l p me, i really don't get this.

Recall that +(-1) = -1.  Thus, +(-12) = -12.  Then you have -6n - 12 + 4n, or
                                                                                               12-4n (answer)
                                      This is equivalent to                          2(2n-6).  

You must also check 4(n-3) - 6n.  Is the end result the same as 2(2n-6)?