Final answer:
The correct property of addition for the provided equation is the Associative Property, which allows for grouping of addition operands in any manner without affecting the sum.
Step-by-step explanation:
The property of addition that justifies the equation in the question is the Associative Property. This property states that when adding three or more numbers, the way in which the numbers are grouped does not affect the sum. In other words, no matter how you group the numbers (which ones you choose to add together first), the total will always be the same. The equation from the question seems to demonstrate this by grouping (4 + 2) together and then adding 6 and (x + 6) to it, which can be regrouped without changing the result of the sum.
To further illustrate, using the Associative Property, we can say that (a + b) + c = a + (b + c). Therefore, according to Equation 2.8, vector addition is associative just as ordinary number addition is. As given in the provided reference, this means for any three vectors A, B, and C, the equation A + (B + C) = (A + B) + C holds true.