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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:

$\left(a\right)\phantom{\rule{0.147em}{0ex}}+\left(-a\right)\phantom{\rule{0.147em}{0ex}}=0.$

Where $$a$$ is an integer, the additive inverse of $$a$$ is $$-a$$.
Example:
$\begin{array}{l}\left(3\right)+\left(-3\right)\phantom{\rule{0.147em}{0ex}}=0.\\ \\ \left(18\right)+\left(-18\right)\phantom{\rule{0.147em}{0ex}}=0.\end{array}$
Important!
1. Additive inverse of $$a$$ is $$-a$$.
2. Addition and subtraction are inverse operations of each other.