How can I draw an arrow on a equation in Tikz by defining points? As in the example.

\documentclass[a4paper]{book}

\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\node at (0,0) {$93  \stackrel{12}{\equiv} - 27 \Rightarrow 93 \stackrel{12}{\equiv} \underbrace{-27 + \overbrace{3 \times 12}}_{9} \Rightarrow 93 \stackrel{12}{\equiv} 9$};
\draw[->] (-.75,.5) -- (-.75,1) -- (1,1) -- (1,.75);
\end{tikzpicture}
\end{document}

How can I draw an arrow on a equation in Tikz by defining points?

Here is a starting code.

\documentclass[a4paper]{book}

\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\node at (0,0) {$93  \stackrel{12}{\equiv} - 27 \Rightarrow 93 \stackrel{12}{\equiv} \underbrace{-27 + \overbrace{3 \times 12}}_{9} \Rightarrow 93 \stackrel{12}{\equiv} 9$};
\draw[->] (-.75,.5) -- (-.75,1) -- (1,1) -- (1,.75);
\end{tikzpicture}
\end{document}