Applying Bernaullis equation at cross-section 1 & 2
P_atm + ρgh + 0 = P₂ + 0 + 0 ⇒ P₂ = P_atm + ρgh    .....(1)
Again applying Bernaullis equation at section 2 & 3
${\mathrm{P}}_2+0+2\mathrm{\rho g}\times \frac{\mathrm{h}}{2}={\mathrm{P}}_{\mathrm{atm}}+\frac{1}{2}2{\mathrm{\rho v}}^2$                .....(2)
$\Rightarrow \mathrm{V}=\sqrt{2\mathrm{gh}}$
This is required velocity of efflux
Applying continuity equation between 3 & 4 cross-section.

$\mathrm{aV}={\mathrm{a}}_1{\mathrm{V}}_1$
This is required velocity of efflux
Applying Bernoullis equation between 3 & 4
${\mathrm{P}}_{\mathrm{atm}}+\frac{1}{2}\left(2\mathrm{\rho }\right){\mathrm{V}}^2+2\mathrm{\rho g}\times \frac{\mathrm{h}}{2}={\mathrm{P}}_{\mathrm{atm}}+\frac{1}{2}\left(2\mathrm{\rho }\right){\mathrm{V}}_1^2+0$
${\mathrm{\rho V}}^2+\mathrm{\rho gh}={\mathrm{\rho V}}_1^2\Rightarrow {\mathrm{V}}_1^2=3\mathrm{gh}$
${\mathrm{a}}_1=\frac{\mathrm{aV}}{{\mathrm{V}}_1}=\frac{\sqrt{6}\cdot \sqrt{2\mathrm{gh}}}{\sqrt{3\mathrm{gh}}}=2{\mathrm{cm}}^2$