Answer
$\color{blue}{[-\frac{3}{2},3]}$
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}
\\&-\frac{3}{2} &\le &x &\le &3
\end{array}
Thus, the solution to the given inequality in interval notation is: $\color{blue}{[-\frac{3}{2},3]}$
