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

%\begin{document}

\begingroup
\renewcommand*{\arraystretch}{1.6}
\[
\boxed{
\begin{array}{rl}
x'(t)  &=-x(t) y(t)^2+x(t)+y(t) \\
y'(t)  &=x(t)^2 y(t)-x(t)-y(t) \\
z'(t)  &=y(t)^2-x(t)^2
\end{array}
}
\]
\endgroup

%\end{document}