This question would be a lot better if you had actually included the exercises. When a question talks about other questions that you already did, you need to include those too. Otherwise we don't know what the question is talking about. Fortunately I was able to figure it out without seeing the exercises, but just for future reference, you should include that extra information.
When two addends(things you add together) are negative, adding them is a lot like adding positive numbers; you just have a negative sign on the sum. For example,
4 + 5 = 9
-4 + -5 = -9
