How to place \blacksquare at the end of a mathematical formula in LaTeX?

Question

At the end of a solution involving a mathematical equation, I like to place \blacksquare at the right hand side corner of the equation. I don't know how to do it. I enclose the code.. Kindly help me on this - thank you!

Thus, the system transfer function is given by
 $$
 G(s) = {X(s) \over U(s)} = {5 \over s^2 + 2 s + 3}   \blacksquare
 $$

Answer 1

There are several possible packages. For example ntheorem.

\documentclass{article}
\usepackage{amsfonts,amsmath,amssymb}
\usepackage[thmmarks]{ntheorem}
{%
\theorembodyfont{\small}
\theoremsymbol{$\square$}
\newtheorem*{Proof}{A label you want}
}
\begin{document}
\begin{Proof}
$G(s) = {X(s) \over U(s)} = {5 \over s^2 + 2 s + 3}$
\end{Proof}
\end{document}

Answer 2

There is now atendofenv package for this.

\documentclass{article}
\usepackage{atendofenv}
\usepackage{amssymb}
\AtEndOfEnv{equation}{$\square$}
\begin{document}
\begin{equation}
 G(s) = {X(s) \over U(s)} = {5 \over s^2 + 2 s + 3}
 \end{equation}
\end{document}