Решите уравнение 7x^2-21=0.

\[7x² - 21 = 0\]

\[7 \cdot \left( x^{2} - 3 \right) = 0\]

\[x^{2} - 3 = 0\]

\[\left( x - \sqrt{3} \right)\left( x + \sqrt{3} \right) = 0\]

\[x - \sqrt{3} = 0\ \ \ \ \ \ \ \ \ x + \sqrt{3} = 0\]

\[x = \sqrt{3}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ x = - \sqrt{3}\]

\[Ответ:x = \sqrt{3};\ x = - \sqrt{3}\]