What is the font used in the $x$ in the following image?

Question

What is the font used in the $x$ in the following image? I have tried many possibilities but cannot reproduce it. Thanks.

Answer 1

It seems that was used \pmb command (stands for "poor man bold") defined in the amsmath package. Instead of it I suggest to use bm package:

\documentclass[margin=3mm]{standalone}
\usepackage{amsmath}
\usepackage{bm}
\begin{document}
$n(\pmb x) = \frac{d\pmb x}{ds}$ \qquad $n(\bm{x}) = \frac{d\bm{x}}{ds}$
\end{document}

Answer 2

That's obviously just a bunch of normal x's overlapped together.

%! TEX program = lualatex
\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\begin{document}
\def\weirdx{x\text{\llap{\raisebox{0.5pt}{$x$}}\llap{\raisebox{0.4pt}{$x$}\hspace{0.5pt}}}}
$n(\weirdx) = \frac{d\weirdx}{ds}$
\end{document}

Although you may want to consider using normal bold italic letter: Math Expression in bold italic - TeX - LaTeX Stack Exchange