\begin{document}
\begin{figure}
\begin{subfigure}{.5\textwidth}
\centering
\includegraphics[height=6cm]{1.eps}
\subcaption{Exact solution}
\end{subfigure}
\ begin {subfigure} {. 5 \ textwidth}
\centering
\includegraphics[height=6cm]{2.eps}
\subcaption{N=2}
\ end {subfigure}
\ begin {subfigure} {. 5 \ textwidth}
\centering
\includegraphics[height=6cm]{3.eps}
\subcaption{N=4}
\end{subfigure}
\ begin {subfigure} {. 5 \ textwidth}
\centering
\includegraphics[height=6cm]{4.eps}
\subcaption{t=1}
\end{subfigure}
\caption{ Exact and estimated solutions with t=1 for Example 1}
\end{figure}
\end{document}