#### Answer

$x=6$

#### Work Step by Step

Square both sides and put all terms on one side to create a quadratic equation. Then use trial and error to factor it, and set the values found equal to zero. One of the solutions does not work.
$\sqrt {3x+18}=x$
$(\sqrt {3x+18})^2=x^2$
$3x+18=x^2$
$x^2-3x-18=0$
$(x+3)(x-6)=0$
$x=-3,6$
$\sqrt {3\times-3+18}\ne-3$
$x=6$