LaTeX.org

Ah, I hope anybody can help me.
I have the following code to input a picture

\begin{figure}
\begin{center}
\includegraphics[width=8cm]{pics/notinward2}
\caption{blablabla}

which works fine.
then I have the following code to make an arrow below the letter x

$\underrightarrow{x}$

which works ...