The additive inverse of something is the number that, if you add it to the original number, you get 0.
The additive inverse of 5, for example, is -5. 5 plus -5 = 0.
You should be able to see how additive inverses are always going to be just the original number with the signs changed. (i.e., times -1)
The additive inverse of -4a would be 4a. -4a + 4a = 0.
