How can I write underneath the equation "b+b+...+b" n-times?

Question

I am new to LaTex, I am using a Overleaf. I'm trying to write the equation

n\times b= b+b+...+b 

with 'n-times' in the bottom.

Answer 1

Is this what you are asking for?

\usepackage{amsmath}
\begin{document}
\begin{equation}
$n \times b=\underbrace{b+b+\ldots+b}_{n}$
\end{equation}
\end{document}

Answer 2

Like this?

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
    n\times b=\underbrace{b+b+\cdots+b}_{n\textup{-times}}
\end{equation}
\end{document}

You might replace \cdots with \ldots. See also When to use \dots, \ldots, \cdots, and blanks? and \dots versus \ldots - is there a difference?. Please read Mico's comment.