The box decelerates at a rate of 3m/s and skids 24 meters becoming to a stop.How fast were you traveling when the box fell out.

V^2 = Vo^2 + 2a*d.

Vo^2 = V^2 - 2a*d
Vo^2 = 0 - (-6)*24 = 144
Vo = 12 m/s.