Re-written, I believe the equation is:
(W+23)+5=W+(23+5)
With this, the parenthesis have nothing multiplying into it, so we can drop the parenthesis to get:
W+23+5=W+23+5
Which is the same thing.
So I'd assume it is the Associative property, because you are combining the like terms of 23 and 5.
Hope this helps!
