check the picture below.
is not very specific above, but sounds like it's asking for an equation for the trapezoid only, mind you, there are square tiles too.
but let's do the trapezoid area then,
[tex]\bf a^{\frac{{ n}}{{ m}}} \implies \sqrt[{ m}]{a^{ n}} \qquad \qquad
\sqrt[{ m}]{a^{ n}}\implies a^{\frac{{ n}}{{ m}}}\\\\
-------------------------------\\\\[/tex]
[tex]\bf A=\cfrac{h(a+b)}{2}\quad
\begin{cases}
A=At\\
a=x\\
h=x\\
b=2x
\end{cases}\implies At=\cfrac{x(x+2x)}{2}
\\\\\\
At=\cfrac{x(3x)}{2}\implies At=\cfrac{3x^2}{2}\impliedby \textit{now, solving for \underline{x}}
\\\\\\
2At=3x^2\implies \cfrac{2At}{3}=x^2\implies \sqrt{\cfrac{2At}{3}}=x\implies \left( \frac{2At}{3} \right)^{\frac{1}{2}}=x[/tex]
