\[x^{2} + 4x - 21 = 0\]
\[x_{1} + x_{2} = - 4;\ \ x_{1} \cdot x_{2} = - 21\]
\[x_{1} = 3;\ \ x_{2} = - 7\]
\[x^{2} + 4x - 21 = (x - 3)(x + 7).\]