\documentclass[11pt]{article}
\usepackage{amsmath}
\begin{document}
first equation
\begin{equation}
  x=1
\end{equation}

Second equation
\par\noindent\fbox{\parbox{\textwidth}{
  \begin{equation}
    x=2\Lambda(x)
  \end{equation}
  Here, $\Lambda(\cdot)$ is \dots}}

\begin{figure}
  \fbox{\parbox{\textwidth}{\[g=3\]}}\caption{Third caption}
\end{figure}

\end{document}