#### Answer

Interval: $(-\infty,\ -2]\cup\ [1,\ \infty)$

#### Work Step by Step

$|2x+1|\geq 3$
We split the absolute value into the positive and negative:
$2x+1\geq 3$
$2x\geq 2$
$x\geq 1$
$2x+1\leq-3$
$2x\leq-4$
$x\leq-2$
Interval: $(-\infty,\ -2]\cup\ [1,\ \infty)$