Final answer:
The step in the simplification uses the Commutative Property of Addition, which allows the order of the elements in the addition to be changed without affecting the sum.
Step-by-step explanation:
The properties of real numbers being used in the simplification 5 + (x + (-5)) = 5 + ((-5) + x) involve rearranging the elements of the addition without changing the overall value.
The particular property used here is the Commutative Property of Addition, which states that the order in which two numbers are added does not affect the sum. This is represented by the equation A + B = B + A. In this case, x and -5 are being switched in their order without affecting the sum.