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

%\begin{document}

\[
\boxed{
\begin{array}{rl}
 (x(t)-y(t)) (x(t)-z(t)) x'(t) &= f(t)\\
 (y(t)-x(t)) (y(t)-z(t)) y'(t) &=f(t)\\
 (z(t)-x(t)) (z(t)-y(t)) z'(t) &=f(t)
\end{array}
}
\]

%\end{document}