Решите уравнение: 7x^2+21x=0.

\[7x^{2} + 21x = 0\]

\[7x(x + 3) = 0\]

\[7x = 0\]

\[x = 0.\]

\[x + 3 = 0\]

\[x = - 3.\]

\[Ответ:x = - 3;x = 0.\]