Question: During preparing a note about elementary row operations, I did the following as similar as I had found through the site:

```
\documentclass[a4paper,12pt]{book}‎
\usepackage{amsmath,amssymb,amsthm,amsfonts}‎ 
\begin{document}‎‎
‎‎\begin{pmatrix}
‎2‎‎ & 1 \\ 
1 & -5\\‎
3 & \circled{7} 
\end{pmatrix}‎
‎‎\xrightarrow ‎{‎\tfrac{-12}{2}‎R_1+ R_3‎\mapsto ‎R_3}‎‎
\begin{pmatrix}
‎2‎‎ & 1 \\ 
1 & -5\\‎
-21 & \circled{5-‎}‎ 
\end{pmatrix}
‎\end{document}‎

```

‎
Is it possible to do it as the picture?

[![enter image description here](https://i.stack.imgur.com/zc0x3.jpg)](https://i.stack.imgur.com/zc0x3.jpg)

I think, If I can do that, they will be illustrated nicely.

Answer: 
Since you didn't provide a complete MWE, I used a `\circled` command found [here](https://tex.stackexchange.com/a/304891/101651).

By the way, the parts already written in your MWE don't exactly correspond to the image, I've replicated the image.

*Edit:* modified to remedy the `Command \scriptsize invalid in math mode` warning pointed out by egreg.

```
\documentclass[a4paper,12pt]{book}
\usepackage{amsmath,amssymb,amsthm,amsfonts}
\usepackage{tikz}
\newcommand*\circled[1]{\tikz[baseline=(A.base)]{\node[circle,draw,inner 
sep=1](A){$#1$};}}% code from: https://tex.stackexchange.com/a/304891/101651
\usepackage{array}
\newcolumntype{R}{>{$\scriptstyle}r<{$}}
\usepackage{booktabs}
\begin{document}
    \[
    \begin{pmatrix}
    2 & 1 \\ 
    1 & -5\\
    3 & \circled{7} 
    \end{pmatrix}
    \xrightarrow[{ 
    \renewcommand*{\arraystretch}{0.8}\begin{tabular}{R@{$\scriptstyle 
    :\;\;$}R@{\hspace{4pt}}R}
        -12R_{1}  &  -24 & -12\\
        R_{3} & 3 & 7\\
        \cmidrule(r){2-3}
        \text{(new) } R_{3} & -21 & -5
        \end{tabular}}]{-12R_1+ R_3\mapsto R_3}
    \begin{pmatrix}
    2 & 1 \\ 
    1 & -5\\
    -21 & \circled{-5} 
    \end{pmatrix}
    \]
\end{document}

```

[![enter image description here](https://i.stack.imgur.com/KyTJy.png)](https://i.stack.imgur.com/KyTJy.png)
@CarLaTeX's answer is very good! However, as an alternative solution, you can also use an array for the "table" below the arrow:

```
\documentclass[a4paper,12pt]{book}
\usepackage{amsmath,amssymb,amsthm,amsfonts}
\usepackage{array}
\begin{document}
\[
\left(\begin{array}{cc}
    2 & 1\\
    1 & -5\\
    3 & 7
\end{array}\right)\xrightarrow[
    \begin{scriptsize}
    \begin{array}{rrr}
        -12R_1: & -24 & -12\\
        R_3: & 3 & 7\\
        \cline{2-3}
        \text{(new) }R_3: & -21 & -5
    \end{array}
    \end{scriptsize}
]{-12R_1+R_3\rightarrow R_3}\left(\begin{array}{cc}
    2 & 1\\
    1 & -5\\
    -21 & -5
\end{array}\right)
\]
\end{document}

```

[![enter image description here](https://i.stack.imgur.com/p8BWO.png)](https://i.stack.imgur.com/p8BWO.png)