\documentclass[12pt]{article}

\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{polyglossia}

\begin{document}

\begin{equation}
    \mathbf{\hat{x}}=\hat{\mathbf{x}}
\end{equation}

\begin{equation}
    \mathbf{\dot{a}}=\dot{\mathbf{a}}
\end{equation}

\begin{equation}
    \mathbf{\vec{r}}=\vec{\mathbf{r}}
\end{equation}

\end{document}