Call the left right-angled triangle's lower left $\;x\;$, so that $\;X=2x\;,\;\;H=\sqrt3x\;$, and thus in the right right-angled triangle we have that the lower leg is $\;3x\;$, and its hypotenuse is $\;2\sqrt3 x\;$.
Adding both lower legs above we get
$$x+3x=4x=12\implies x=3\implies \begin{cases}X=6\\\\{}\\\Y=2\sqrt3\cdot3=6\sqrt3\end{cases}$$
and the above only uses basic Euclidean geometry.