[Tex/LaTex] side by side subfigures alignment problem

Question

I am new in latex writing. I am have problem with figures alignment. I am trying to put six figures on single page side by side. but the last figure is bit right shifted. I am no getting what could be the reason behind this. Please help me out to fix this problem. Thank you. Here is my code,

\begin{figure}[h]
            \begin{subfigure}[h]{7cm}
                \centering
                \includegraphics[scale=0.38]{images/er1.pdf}
                \caption{PE}
                \label{snr_a}
            \end{subfigure}             
            \begin{subfigure}[h]{7cm}
                \centering
                \includegraphics[scale=0.38]{images/sb.pdf}
                \caption{Pn}
                \label{snr_b}   
            \end{subfigure}
            \begin{subfigure}[h]{7cm}   
                \centering      
                \includegraphics[scale=0.38]{images/s5.pdf}
                \caption{Pl}
                \label{snr_c}
            \end{subfigure}
            \begin{subfigure}[h]{7cm}   
                \centering
                \includegraphics[scale=0.38]{images/s0.pdf}
                \caption{Pc}
                \label{snr_d}
            \end{subfigure}
            \begin{subfigure}[h]{7cm}   
                \centering
                \includegraphics[scale=0.38]{images/s5.pdf}
                \caption{Pd}
                \label{snr_e}
            \end{subfigure}
            \begin{subfigure}[h]{7cm}   
                \centering
                \includegraphics[scale=0.38]{images/s9.pdf}
                \caption{Pb}
                \label{snr_f}
            \end{subfigure}
            \caption{abcdef}
            \label{os}
        \end{figure}    

Answer 1

Don't use the [h] option alone, use [htp]. Second, the [h] option doesn't make sense for subfigure (for which the same options as for minipage apply, that is, [t] or [b], where the absence is equivalent to [c]).

It's also better to specify the width of figures, rather than a scale factor. In order to be independent of the actual text width, use a multiple of \textwidth, which is reset in a subfigure to the stated width.

The figure environment has a global \centering, which avoids the filling in the last line (it's the same as for normal paragraphs, otherwise, where the last line is filled). Two subfigures are placed side by side, with a width slightly less than half of the text width and \hfill between them. A blank line separates the rows. Adjust to suit your tastes.

I the example, for lack of your images, I also specify a height, but you shouldn't. Also the demo option is just for producing the example, don't use it yourself.

\documentclass{article}
\usepackage{showframe} % just for testing
\usepackage[demo]{graphicx} % remove the 'demo' option in the production version
\usepackage{subcaption}

\begin{document}

\begin{figure}[htp]
\centering
\begin{subfigure}{0.49\textwidth}
  \centering
  \includegraphics[width=\textwidth,height=2cm]{images/er1.pdf}
  \caption{PE}
  \label{snr_a}
\end{subfigure}\hfill
\begin{subfigure}{0.49\textwidth}
  \centering
  \includegraphics[width=\textwidth,height=2cm]{images/sb.pdf}
  \caption{Pn}
  \label{snr_b}   
\end{subfigure}

\begin{subfigure}{0.49\textwidth}   
  \centering      
  \includegraphics[width=\textwidth,height=2cm]{images/s5.pdf}
  \caption{Pl}
  \label{snr_c}
\end{subfigure}\hfill
\begin{subfigure}{0.49\textwidth}   
  \centering
  \includegraphics[width=\textwidth,height=2cm]{images/s0.pdf}
  \caption{Pc}
  \label{snr_d}
\end{subfigure}

\begin{subfigure}{0.49\textwidth}   
  \centering
  \includegraphics[width=\textwidth,height=2cm]{images/s5.pdf}
  \caption{Pd}
  \label{snr_e}
\end{subfigure}\hfill
\begin{subfigure}{0.49\textwidth}   
  \centering
  \includegraphics[width=\textwidth,height=2cm]{images/s9.pdf}
  \caption{Pb}
  \label{snr_f}
\end{subfigure}
\caption{abcdef}
\label{os}

\end{figure}

\end{document}