#### Answer

$\color{blue}{-1.5 \le x \le 3}$

#### Work Step by Step

Subtract $3$ to each part:
\begin{array}{ccccc}
&-6-3 &\le &6x+3-3 &\le &21-3
\\&-9 &\le &6x &\le &18
\end{array}
Divide each part by $6$:
\begin{array}{ccccc}
\\&\frac{-9}{6} &\le &\frac{6x}{6} &\le &\frac{18}{6}
\\&-1.5 &\le &x &\le &3
\end{array}
Thus, the solution to the given inequality is:
$\color{blue}{-1.5 \le x \le 3}$