How could I bring the highlighted text to the very next line with a left-alignment?

Question

\documentclass[11pt,a4paper]{article}
\usepackage{blindtext}
\usepackage{mathtools}
\usepackage{amsmath}
\usepackage{IEEEtrantools}


\begin{document}
Proof:
\vspace{-1em}
\begin{IEEEeqnarray*}{rCl}
E&=&\times \underbrace{(\times a\times a\times a\times \dotsm)}_{m\,\text{factors}}\div \underbrace{(\times a\times a\times a\times \dotsm)}_{n\,\text{factors}};\,\text{(Definition of an index)}\\
&=&\times a\times a\times a\times \dotsm \times a \div a\div a\div a \dotsm \div a;\,\text{(The law of association for multiplication and division)}\\
&=&\times (\times a\times a\times a\times \dotsm m-n\,\text{factors})\\
&&\times (a\div a)\times (a\div a)\times \dotsm n\,\text{factors};\,\text{(The laws of commutation and association)}\\    
&=&\times \underbrace{(\times a\times a\times a\times \dotsm)}_{m-n\,\text{factors}};\,\text{(Properties of division)}\\
E&=&a^{m-n}.\,\text{(Definition of an index)}
\end{IEEEeqnarray*}
[![enter image description here][1]][1]\end{document} 

Answer 1

Since IEEEtranstools is not standard LaTeX, I show it below without it. I just move the comment to its own row, and \rlap it, followed by a phantom E.

EDITED to add extra empty groups {} to make the \times a binary operator, as needed.

As Runar points out in a comment, in the absence of IEEEtranstool, the eqnarray method shown below would not be preferred, but rather one of the align environments of amsmath. So below, I show both approaches.

\documentclass[11pt,a4paper]{article}
\usepackage{blindtext}
\usepackage{mathtools}
\usepackage{amsmath}
%\usepackage{IEEEtrantools}

\begin{document}
Proof:
\vspace{-1em}
%\begin{IEEEeqnarray*}{rCl}
\begin{eqnarray*}
E&=&\times \underbrace{(\times a\times a\times a\times \dotsm)}_{m\,\text{factors}}\div \underbrace{(\times a\times a\times a\times \dotsm)}_{n\,\text{factors}};\,\text{(Definition of an index)}\\
&=&{}\times a\times a\times a\times \dotsm \times a \div a\div a\div a \dotsm \div a;\\
 \rlap{(The law of association for multiplication and division)}\phantom{E}& \\
&=&{}\times (\times a\times a\times a\times \dotsm m-n\,\text{factors})\\
&&{}\times (a\div a)\times (a\div a)\times \dotsm n\,\text{factors};\,\text{(The laws of commutation and association)}\\    
&=&{}\times \underbrace{(\times a\times a\times a\times \dotsm)}_{m-n\,\text{factors}};\,\text{(Properties of division)}\\
E&=&a^{m-n}.\,\text{(Definition of an index)}
\end{eqnarray*}
%\end{IEEEeqnarray*}

\begin{align*}
E&=\times \underbrace{(\times a\times a\times a\times \dotsm)}_{m\,\text{factors}}\div \underbrace{(\times a\times a\times a\times \dotsm)}_{n\,\text{factors}};\,\text{(Definition of an index)}\\
&={}\times a\times a\times a\times \dotsm \times a \div a\div a\div a \dotsm \div a;\\
 \rlap{(The law of association for multiplication and division)}\phantom{E}& \\
&={}\times (\times a\times a\times a\times \dotsm m-n\,\text{factors})\\
&\phantom{{}={}}{}\times (a\div a)\times (a\div a)\times \dotsm n\,\text{factors};\,\text{(The laws of commutation and association)}\\    
&={}\times \underbrace{(\times a\times a\times a\times \dotsm)}_{m-n\,\text{factors}};\,\text{(Properties of division)}\\
E&=a^{m-n}.\,\text{(Definition of an index)}
\end{align*}
\end{document}