I 've used tabbing environment instead of align* environment in order to align all equations inside all align environment together as suggested in that answer for another:
https://tex.stackexchange.com/questions/120104/collectively-aligning-multiple-align-environments
I've also used the command \tagthisline
to number specified equations, and used the following code in the preamble as mentioned in the above answer:
newcommand{\tagthisaux}{\refstepcounter{equation}(\theequation)}
\newcommand{\tagthisline}{\tagthisaux}
It gives a number for the equation, but i cant label it, whenever i use the command label, or putting the label key inside two curly braces {} beside the command \tagthisline{...}
the referring code also appears beside the equation.
\documentclass[12pt]{article}
\usepackage{siunitx}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{amssymb}
\newcommand{\tagthisaux}{\refstepcounter{equation}(\theequation)}
\newcommand{\tagthisline}{\tagthisaux}
\begin{document}
\begin{tabbing} \hspace{3cm} \= \hspace{3cm} \= \hspace{.5cm} \= \hspace{3cm} \=
\kill
\> \(Ra_L\) \> \(=\) \> \(Gr\cdot\Pr\)\\
\>\(\)\> \(=\) \> \(\frac{g\beta\,\Delta TL_c^3}{\mathcal{V}^2}\cdot\,Pr\) \tagthisline{ral}
\end{tabbing}
\end{document}