# Equation + explanation below

I have to write an explanation to my equation, and I would like to do it, below the equation. But, I do not know how to do that.

It should look like this:

My preamble look like this:

\documentclass[a4paper,12pt,oneside]{article}
\usepackage[english]{babel}
\usepackage{tikz}
\usepackage{pgf}
\usetikzlibrary{shapes,arrows,positioning,calc}
\usepackage{tabu}
\usepackage{hyperref}
\usepackage{longtable}
\usepackage{array}
\usepackage{siunitx,multirow,tabularx,booktabs}
\usepackage{amsmath,amssymb,bm,mathtools}

\begin{document}

$$E = \alpha X + \beta F + u$$

\end{document}


To achieve what you want, I would recommend using a list type environment such as itemize from the enumitem package, along with the varwidth package:

## Code:

\documentclass[a4paper,12pt,oneside]{article}
\usepackage[english]{babel}
\usepackage{tikz}
\usepackage{pgf}
\usetikzlibrary{shapes,arrows,positioning,calc}
\usepackage{tabu}
\usepackage{hyperref}
\usepackage{longtable}
\usepackage{array}
\usepackage{siunitx,multirow,tabularx,booktabs}
\usepackage{amsmath,amssymb,bm,mathtools}

\usepackage{enumitem}
\usepackage{varwidth}

\begin{document}

$$E = \alpha X + \beta F + u$$
\begin{center}
\begin{varwidth}{\linewidth}
\begin{itemize}[nosep]
\item [$E$] is xxx
\item [$X$] is yyy
\end{itemize}
\end{varwidth}
\end{center}

\end{document}