Theory:

Adding the opposite of a number to the same number will equal to \(0\) (zero).
Inverse property of addition: For every integer (\(a\)) there exists another opposite integer (\(-a\)) such that:
 
(a)+(a)=0.
 
Where \(a\) is an integer, the additive inverse of \(a\) is \(-a\).
Example:
(4)+(4)=0.(15)+(15)=0.
Important!
1. Additive inverse of \(a\) is \(-a\).
2. Addition and subtraction are inverse operations of each other.