We have
 breadth of hall = b = 15 m
 height of hall = h = 8 m.
 let length of hall be ℓ
 Then 
  area of 4 walls of hall = 2h (ℓ + b) = 1068 m²
⇒    2h (ℓ + b) = 1068 m²
⇒    $\mathrm{\ell }=\frac{1068}{2\mathrm{h}}-\mathrm{b} \Rightarrow \mathrm{\ell }$
$=\frac{1068}{2\times 8}-15 \Rightarrow \frac{531}{8}-15 \Rightarrow \frac{531-120}{8}=\frac{411}{8}\mathrm{m}$