If you really want something like that one way would be something like this:
\documentclass{article}
\begin{document}
\subsection{CLOSED :}
If the operator results in the same set that it operates upon , then the
algebraic system is called ' CLOSED ' .
\vspace{1\baselineskip} \noindent
Example :
\vspace{1\baselineskip} \noindent
1. Set of an integer is closed under + , - , $\times$ operator.
\vspace{1\baselineskip} \noindent
2. Set of an integer is not closed under / operator.
\vspace{1\baselineskip} \noindent
Let A = \{ $\alpha$,$\beta$,$\psi$ \}
\end{document}
However, I would probably write it like this:
\documentclass{article}
\begin{document}
\subsection{CLOSED :}
If the operator results in the same set that it operates upon, then the
algebraic system is called `CLOSED'.
\vspace{\baselineskip}\noindent
Example:
\begin{enumerate}
\item Set of an integer is closed under +, -, $\times$ operator.
\item Set of an integer is not closed under / operator.
\end{enumerate}
Let A = \{$\alpha$, $\beta$, $\psi$\}
\end{document}
Or maybe define something more fancy for the Example.