TeX source:
\begin{array}{rcl} A_O &=& a^2+2ah_s \cr A_O &=& a^2+2a\sqrt{h^2+\left(\dfrac{a}{2}\right)^2} \cr A_O &=& (7 \, m)^2+2\cdot 7 \, m \cdot \sqrt{(5 \, m)^2+\left(\dfrac{7}{2} \, m \right)^2} \cr A_O &\approx& 134{,}45\, m^2 \end{array}