The length of a hall is 18 m and the width 12 m.

Length of the hall $=18 \mathrm{~m}$

Its width $=12 \mathrm{~m}$

Suppose that the height of the wall is $\mathrm{h} \mathrm{m}$.

Also, sum of the areas of the floor and the flat roof $=$ sum of the areas of the four walls

$\Rightarrow 2 \times($ length $\times$ breadth $)=2 \times($ length $+$ breadth $) \times$ height

$\Rightarrow 2 \times(18 \times 12)=2 \times(18+12) \times \mathrm{h}$

$\Rightarrow 432=60 \times \mathrm{h}$

$\Rightarrow \mathrm{h}=\frac{432}{60}=7.2 \mathrm{~m}$

$\therefore$ The height of wall is $7.2 \mathrm{~m} .$