%\documentclass[12pt]{article}
%\usepackage{amsmath,mathtools}

%\begin{document}

\begingroup
\renewcommand*{\arraystretch}{1.5}
\[
\boxed{
\begin{array}{rl}
x''(t) &= \frac{\partial F}{\partial x}\\
y''(t) &= \frac{\partial F}{\partial y}\\
z''(t) &= \frac{\partial F}{\partial z}
\end{array}
}
\]
\endgroup

Where $F=F(r)$ and $r=\sqrt{x(t)^2+y(t)^2+z(t)^2}$

%\end{document}