LaTeX.org

you could superscript the minus sign as follows:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\bar{s}_{ap}^{^-1}\cdot\bar{a} = \bar{v}_{ap}
\end{equation}
\begin{align}
\bar{s}_{ap}^{-1}\cdot\bar{a} &= \bar{v}_{ap}\\
\bar{s}_{gt}^{-1}\cdot\bar{a} &= \bar{v ...