the longest line segment that can be drawn in a right rectangular prism that is 14 cm long, 11 cm wide and 8 cm tall is 3D diagonal from one corner/vertex "at the floor" to the opposite corner/vertex "at the ceiling". Its length is, applying the Pythagorean theorem in 3D is L == = 19.519 centimeters. It is the longest diagonal" in this 3D box (= prism).