\documentclass{article}
\usepackage{amsmath}     % Using a mathematical package
\begin{document}

This is the beginning of the article.
\section{first section}
Text of the first section

\subsection{First subsection}
In this subsection I am going to present an
equation.   Here it is.
\begin{equation}
   \phi^{244}_{\bigcirc} =
  10^{-7} C^{244}\rho _{sed}r_{sed} <
  0.06\;{\textstyle\; ^{244}Pu atoms/cm^2 yr}.
 \end{equation}

Now here's another equation.   I have numbered it 3.45
$$
d^2\sigma = \frac{e^2_n q^3 U^2(q)}{3\pi^2 c^3 p^2}
\frac{d\omega }{\omega } dq =
 \frac{e^2_n q^3 U^2(q)}{3\pi^2 c^3 v^2 m^2_n}
\frac{d\omega }{\omega } dq.
\eqno(3.45)
$$

\end{document}