I have a chain of equations in AMS align environment in LyX, and I want to cite numbered equations as justification for some of my steps in the chain (see the picture).
I don't know if there's a standard way to do this. My current method is to introduce horizontal space and then to cite the reasoning to the right. However, when I do this the displayed chain is pushed to the left (hence no longer centered) and the amount of horizontal spacing needed changes dramatically. Is there a uniform way to justify steps on the right of the chain while keeping the chain centered? If there's a more standard way to justify steps in a chain, I'm open to suggestions. Thanks in advance.
MWE:
\documentclass[english,reqno]{amsart}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{amstext}
\usepackage{amsthm}
\usepackage{setspace}
\makeatletter
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands.
\numberwithin{figure}{section}
\makeatother
\usepackage{babel}
\begin{document}
\begin{singlespace}
\begin{align*}
A= & B\\
= & C\qquad\qquad\text{by \ensuremath{\left(3\right)}}
\end{align*}
\end{singlespace}