[Tex/LaTex] Referencing equations in latex

Question

I am trying to reference equations in latex (I am writing in overleaf) and am using the code as in the official overleaf documentation i.e.

\begin{equation}\label{Emc2}
E=mc^2
\end{equation}

Einstein wrote his famous equation \ref{Emc2}, blah blah blah ....

However on compiling, the reference just shows as a pure number i.e.

Einstein wrote his famous equation 1, blah blah blah ….

whereas I would like the number reference in parentheses, as it is in most academic papers

Einstein wrote his famous equation (1), blah blah blah ….

I am surprised that I haven't been able to find documentation for this.

EDIT:
As requested, I am putting in more complete source code. I am writing in several files and have a control file:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{subfiles}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{a4wide}

\title{   }
\author{   }
\date{April 2019}

\begin{document}
\maketitle

\subfile{introduction.tex}
\subfile{math.tex}
\subfile{1.tex}
\subfile{2.tex}
\subfile{3.tex}
\subfile{4.tex}
\subfile{5.tex}

\bibliographystyle{unsrt}
\bibliography{ref.bib}

\end{document}

And an example of where I am putting in the equation and reference

\documentclass[1.tex]{subfiles}
\begin{document}

\section{.....}

..text..

\begin{equation}\label{PLT}
Z(\lambda) = \int_{\mathcal{C_x}} dz g(z) e^{-\frac{f(x)}{\lambda}}
\end{equation}

..text..

iv) The exact value of the integral \eref{PLT} is then given by...

Which shows up as

The exact value of the integral PLT is then given by…

Note that when I am compiling, I am doing so from my control file. All of the citations come out fine in squared brackets, and the references work when using \ref, but not when using \eref

Answer 1

You have two options; either use (\ref{..}) or \eqref{..} as noted out by @marmot in a comment. The latter requires adding amsmath in your preamble.

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath}   % <-- for \eqref
\begin{document}

\begin{equation}\label{Emc2}
E=mc^2
\end{equation}

Einstein wrote his famous equation (\ref{Emc2}), blah blah blah ....

Einstein wrote his famous equation \eqref{Emc2}, blah blah blah ....

\end{document}