Answer
{$-12,-1$}
Work Step by Step
Since, $u^2+7u-18=0$
Factorize the expression as follows: $(u+9)(u-2)=0$
or, $u=${$-9,2$}
Replace $u$ with $x+3$, we have
$x+3=-9 \implies x=-12$
and $x+3=2$
or, $x=-1$
Hence, our solution set is $x=${$-12,-1$}