$\begin{array}{l} \sqrt{\sqrt5 - \sqrt3x} = \sqrt{8 + 2\sqrt{15}}\\ ĐKXĐ: \,\,\sqrt{5} - \sqrt{3}x \geq 0\Leftrightarrow x \leq \dfrac{\sqrt{15}}{3}\\ \sqrt{\sqrt5 - \sqrt3x} = \sqrt{8 + 2\sqrt{15}}\\ \Leftrightarrow \sqrt5 - \sqrt3x = 8 + 2\sqrt{15}\\ \Leftrightarrow x = \dfrac{\sqrt5 - 8 - 2\sqrt{15}}{\sqrt3} = \dfrac{\sqrt{15} - 8\sqrt3 - 6 \sqrt5}{3} \end{array}$
