\[5x - 18\sqrt{x} - 8 = 0\]
\[Сделаем\ замену\ \ t = \sqrt{x}:\]
\[5t^{2} - 18t - 8 = 0\]
\[D = 324 + 160 = 484 = 22^{2}\]
\[t_{1,2} = \frac{18 \pm 22}{10} = 4;\ - 0,4\]
\[\sqrt{x} = 4\ \ \ \ \ \ \ \ \ \ \sqrt{x} = - 0,4\]
\[x = 16\ \ \ \ \ \ \ \ \ \ \ \ \varnothing\ \]
\[Ответ:\ 16.\]