\[15 - 5x^{2} = 0\]
\[5 \cdot \left( 3 - x^{2} \right) = 0\]
\[3 - x^{2} = 0\]
\[x^{2} = 3\]
\[x = \pm \sqrt{3}.\]
\[Ответ:x = \pm \sqrt{3}.\]