Answer
$x=\left\{ -7,3 \right\}$
Work Step by Step
Factoring the given equation, $
x^2+4x-21=0
,$ results to
\begin{array}{l}\require{cancel}
(x+7)(x-3)=0
.\end{array}
Equating each factor to zero (Zero Product Principle), then the solutions to the equation, $
(x+7)(x-3)=0
,$ is
\begin{array}{l}\require{cancel}
x+7=0
\\\\
x=-7
,\\\\\text{OR}\\\\
x-3=0
\\\\
x=3
.\end{array}
Hence, $
x=\left\{ -7,3 \right\}
.$
