|AD| = 30 - 6 = 24
The triangle ABC and the triangle BDA are similar, therefore:
[tex]\dfrac{30}{x}=\dfrac{x}{24}\ \ \ \ |cross\ multiply\\\\x^2=30\cdot24\to x=\sqrt{30\cdot24}\\\\x=\sqrt{5\cdot6\cdot6\cdot4}\\\\x=\sqrt{6^2\cdot4\cdot5}\\\\x=\sqrt{6^2}\cdot\sqrt4\cdot\sqrt5\\\\x=6\cdot2\cdot5\\\\\boxed{x=12\sqrt5}[/tex]
