$x\geq12$ or $x\leq-18$.
Work Step by Step
$|x+3|\geq15$ implies $x+3\geq15\\x\geq12$ or $x+3\leq-15\\x\leq-18$. Thus $x\geq12$ or $x\leq-18$. Then, by using a graphing utility, the solution set is attached.
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