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

\begingroup
\renewcommand*{\arraystretch}{1.8}
\[
\boxed{
\begin{array}{rl}
x_1'(t) \sin (x_2(t))           &= x_4(t) \sin (x_3(t))+x_5(t) \cos (x_3(t))\\
x_2'(t)                         &= x_4(t) \cos (x_3(t))-x_5(t) \sin (x_3(t))\\
x_1'(t) \cos (x_2(t))+x_3'(t)   &= a\\
x_4'(t)-a (1-\lambda ) x_5(t)   &= -m \sin (x_2(t)) \cos (x_3(t))\\
a (1-\lambda ) x_4(t)+x_5'(t)   &= m \sin (x_2(t)) \sin (x_3(t))
\end{array}}
\]
\endgroup

%\end{document}