It says that [itex]r_{n}[/itex] and [itex]r_{a}[/itex] give the position with respect to the origin, while r is a relative position vector.
So what vectors would [itex]r_{n}[/itex] and [itex]r_{a}[/itex] be if they start at the origin? Where would the end of those vectors lie? Given that [itex]r = r_{n} - r_{a}[/itex].
If the answer isn't obvious then review material on graphically subtracting vectors.