What property is illustrated? (x + y) + z = x + (y + z)

Associative property of addition.

Step-by-step explanation:

This looks to be the associative property of addition. Pretty sure.

This was explained to me as:

1 + (2 + 4) = (1 + 2) + 4

or

A + (B + C) = (A + B) + C

For further reference on addition properties note:

Commutative property is A + B = B + A

Identity property is A + 0 = A

Commutative property

Explanation: repetitive numbers