\[D = ( - 4)^{2} - 4 \cdot 1 \cdot ( - 21) =\]
\[= 16 + 84 = 100\]
\[x_{1} = \frac{4 + \sqrt{100}}{2 \cdot 1} = \frac{4 + 10}{2} =\]
\[= \frac{14}{2} = 7\]
\[x_{2} = \frac{4 - \sqrt{100}}{2 \cdot 1} = \frac{4 - 10}{2} =\]
\[= - \frac{6}{2} = - 3\]
\[Ответ:x = 0;\ \ x = 36;\ \ x = \pm 7;\ \]
\[x = - 3.\]