I assume the length of your room is 6 meters longer than its width.
If thats the case, set the equation as
2(x+6)+2(x)=24
2x+12+2x=24
4x+12=24
4x=12
x=3 The width is 3 and we know the length is 6 meters longer than it.
So the length can be expressed as x+6=3+6=9
So the missing dimension is 9 meters.