From the question given, we can observe that the sum of the two numbers on the left equals zero.
When the sum of two real numbers is zero, then each real number is said to be the additive inverse of the other. So, we have R + (-R) = 0, where R is a real number. R and -R are the additive inverses of each other.
In a nutshell, The additive inverse property states that a number is added to its opposite number will result in zero
Thus, the property used is Additive Inverse Property.