\[x² + 4x - 21 > 0\]
\[x^{2} + 4x - 21 = 0\]
\[x_{1} + x_{2} = - 4,\ \ x_{1} = - 7\]
\[x_{1} \cdot x_{2} = - 21,\ \ x_{2} = 3\]
\[Ответ:( - \infty; - 7) \cup (3;\ + \infty).\]