\[\sqrt{x^{2}} = \sqrt{2\sqrt{10} - 6} \cdot \sqrt{2\sqrt{10} + 6}\]
\[\sqrt{x^{2}} = \sqrt{40 - 36}\]
\[\sqrt{x^{2}} = \sqrt{4}\]
\[x^{2} = 4\]
\[x = \pm 2.\]