Answer
$x=3$ and $x=-7$
Work Step by Step
The given equation is
$\Rightarrow x^2+4x-21=0$
Find the value of $(\frac{b}{2})^2$.
Substitute $4$ for $b$.
$=(\frac{4}{2})^2$
Simplify.
$=(2)^2$
$=4$
Add $25$ to each side of the equation.
$\Rightarrow x^2+4x-21+25=0+25$
Simplify.
$\Rightarrow x^2+4x+4=25$
Write the left side as the square of a binomial.
$\Rightarrow (x+2)^2=25$
Take the square root of each side.
$\Rightarrow x+2=\pm5$
Subtract $2$ from each side.
$\Rightarrow x+2-2=\pm5-2$
Simplify.
$\Rightarrow x=\pm5-2$
The solutions are $x=5-2=3$ and $x=-5-2=-7$.
