Which property does the equation illustrate (x+5) + 9=x + (5+9)

This equation illustrates the commutative property. It illustrates this because it shows that addition works no matter what order the numbers are in. This also works for multiplication.

Two properties have been illustrated:

The commutative property, whereby the order in which the terms are arranged will not affect the result.

The associative property, as the way in which the terms are grouped won't change the result.