How fast are the pebbles going when they hit her window.?

Romeo is chucking pebbles gently up to Juliet's Window, and he wants the pebble to hit the window with only a horizontal component of velocity. He is standing at the edge of a rose garden 4.5 m below her window and 5.0 m from the base of the wall. How fast are the pebbles going when they hit her window?