This question was uploaded on 31/12/21 on social media accounts.
Solution:
There are two similar triangles:
By taking ration of their small sides and hypotanuse:
\[\frac{9-a}{9+a} = \frac{a-3}{a+3}\]
\[\Rightarrow a^2 = 27\]
\[\Rightarrow a = 3\sqrt3\]
\[\Rightarrow x = 2a = 6\sqrt3\]
