As you add negative numbers together, the answer moves further to the left along the number line, so when adding two negative numbers, it's straight addition and the answer is always (also) a negative number.
8 + -4 + -8 = -4
Negative 4
- 4 + (- 4) = - 8
-8
negative 8
The sum is negative 8.
(-4) + (+8) is +4
12
+8 plus -4 is essentially the same as 8 - 4. Therefore the answer is four.
-8 + 4 - (-9) = -8 + 4 + 9 = -8 + 13 = 5.
12
-4