With negative numbers you have to think of them on a number line... 0 in the middle, if you have a positive number you go right on the number line. If you have a negative number you go left.
So, for example...
(-14) + (-6)
you are starting at -14 on the number line then adding a negative 6, so you go 6 more places to the left on the number line and now you are at -20.
then -20 + 6
Now you start at -20 and add 6 so you move 6 places to the right on the number line, so you are back to -14.
(-14) + (-6) + 6=
-20 + 6 =
-14