Answer
$- \frac{3}{2}$ $\leq$ $x$ $\leq$ $3$
The equivalent interval notation is $[-\frac{3}{2}, 3]$.
Work Step by Step
$-6$ $\leq$ $6x + 3$ $\leq$ $21$
$-6 - 3$ $\leq$ $6x$ $\leq$ $21 - 3$
$- \frac{9}{6}$ $\leq$ $x$ $\leq$ $\frac{18}{6}$
$- \frac{3}{2}$ $\leq$ $x$ $\leq$ $3$
The equivalent interval notation is $[-\frac{3}{2}, 3]$.