You have a choice between a six-pointed asterisk, generated by \ast, and a five-pointed asterisk, generated by \star. Both of these macros need to be used in math-mode; choose whichever symbol you like better.
I would also enlarge the floor-related symbols of the "outer" \floor directive slightly as a visual aid towards parsing the equation.

\documentclass{article}
\usepackage{mathtools} % for \DeclarePairedDelimiter macro
\DeclarePairedDelimiter{\floor}{\lfloor}{\rfloor}
\setlength\textwidth{5cm} % just for this example
\begin{document}
either
\[
\floor[\big]{7(n\pi - \floor{n\pi})} \tag{$\star$}
\]
or
\[
\floor[\big]{7(n\pi - \floor{n\pi})} \tag{$\ast$}
\]
\end{document}
$$...$$... it's deprecated. Use\[...\]... which is robust meanwhile since TL2015 – Jul 07 '15 at 11:05