How can I justify without making a new line in mdframed?

Question

 \begin{mdframed}[backgroundcolor=green]     
 \textbf{Theorem:}     
 \begin{center} A set $E$ is {\bf open} $\iff E^c$ is {\bf closed} \end{center}      
 \centerline{or equivalently,} 
 \centerline{A set $E$ is {\bf open} $\iff E^c$ is {\bf closed} }
\end{mdframed}

So basically I want to get Theorem to align on the same line as \begin{center} A set $E$ is {\bf open} $\iff E^c$ is {\bf closed} \end{center} without producing another new line. Adding flushleft inside the center environment has no effect.

Answer 1

Try using only center and then some box-control (via \phantom) for the first line:

\documentclass{article}
\usepackage{xcolor,mdframed}% http://ctan.org/pkg/{xcolor,mdframed}
\begin{document}
% Original layout/use
\begin{mdframed}[backgroundcolor=green]
  \textbf{Theorem:}
  \begin{center} A set $E$ is {\bf open} $\iff E^c$ is {\bf closed} \end{center}
  \centerline{or equivalently,}
  \centerline{A set $E$ is {\bf open} $\iff E^c$ is {\bf closed} }
\end{mdframed}

\bigskip

% Updated layout/use
\begin{mdframed}[backgroundcolor=green]
\begin{center}
  \textbf{Theorem:} \hfill A set~$E$ is \textbf{open} $\iff E^c$ is \textbf{closed} \hfill \phantom{\textbf{Theorem:}} \\
  or equivalently, \\
  A set~$E$ is \textbf{open} $\iff E^c$ is \textbf{closed}
\end{center}
\end{mdframed}

\end{document}

The first line is set at full width using a displayed Theorem: on the left, and a \phantom Theorem: on the right.