A slingshot fires a pebble from the top of a building....?

If the pebble is shot straight horizontally in the beginning, then the horizontal speed would be 17.7 m/s. This is because in the horizontal direction, there is no acceleration. Therefore, there is no change in speed in that direction. I guess you got parts b and c b/c you didn't ask about that. Still, I got 26 m/s for those ( I don't really understand the down and up part, so may be it means positive and negative). I'm guessing this is for webign? Hope this helps :)