18. A moving circle is externally tangent to $(x+2)^{2}+y^{2}=4$ and tangent to the line $x=2$. Then, the equation of the locus of the center of the moving circle is
$\begin{array}{l}\text { 18. } y^{2}+12 x-12 \\ =0\end{array}$

The translation is as follows:

$\begin{array}{l}\text { 18. } y^{2}+12 x-12 \\ =0\end{array}$

Since the text is a mathematical equation, it remains the same in English.