A stone is thrown up with a velocity ‘U’ from the top of the tower and reaches ground with a  velocity 2U the height of tower is

we know, V2−U2=2aSby question , final velocity= V=2U;acceleration due to gravity=-g;height of the tower=-h  (negative since ground is below point of projection)V2−U2=2−g−h4U2−U2=2gh⇒3U2=2ghh=3U22g