So you have two of those--when you add those together, you get "seven and eight-tenths," or 7.8 The 2 shorter sides are what's left to still measure. If the total perimeter is 10.4, you get the shorter sides by: 10.4 - 7.8 = 2.6, but REMEMBER, the 2.6 is BOTH shorter sides added together, so each ONE of them is 1.3 SO... your hallway/rectangle is 3.9 meters on each long side, and 1.3 meters on each short side. i hope this helps you