The associative property of addition is characterized by altering the order of operations by putting brackets in a different place. The answer will always be the same. Just the order is different.
So one possibility and the one you are likely expected to give is
x + (y + 17)
That means that if 17 and y can be added by giving y a value, then they will be done first.
Then you add z to the result if it has been given a value.
The answer you are expected to give is x + (y + 17)
