Answer
$x=2$
Work Step by Step
The given function is
$\Rightarrow f(x)=6x+9$
Substitute $21$ for $f(x)$.
$\Rightarrow 21=6x+9$
Subtract $9$ from each side.
$\Rightarrow 21-9=6x+9-9$
Simplify.
$\Rightarrow 12=6x$
Divide each side by $6$.
$\Rightarrow \frac{12}{6}=\frac{6x}{6}$
Simplify.
$\Rightarrow 2=x$.
