\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
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\begin{document}
\begin{equation*}
k=4\pi\frac{e}{h} \text{V}^{-1}\text{s}^{-1}= 2\frac{e}{h}
  \text{Hz/V$^{-1}$}, 
\end{equation*}
\end{document}