Finally $1 / 3$ rd of the water spilled out So volume of water spilled out finally
$=V f=$$\frac{2 \tan \theta \times A}{2}=\frac{L^{3} \tan \theta}{2}$
this is $1 / 3$ volume of $L^{3}$
$\Rightarrow \frac{\tan \theta}{2}=\frac{1}{3} \Rightarrow \tan \theta=2 / 3=\mathrm{a} / \mathrm{g}$
$a=2 g / 3$