A youngster on the roof of a 21.0-m-tall building stands at the edge and throws a paper plane from a height of?

A youngster on the roof of a 21.0-m-tall building stands at the edge and throws a paper plane from a height of 1.21 m above the roof. It sails around before landing directly in front of him 18.0 m from the building. What is the magnitude of the displacement of the plane from its launch point?