%% formulation
\documentclass{standalone}
\usepackage{amsmath}
\usepackage{bm}
\usepackage{graphicx}
\usepackage{xcolor}
\begin{document}
\setlength{\abovedisplayskip}{2pt}
\setlength{\belowdisplayskip}{2pt}
\pagecolor{white}
\scalebox{2}{\parbox{0.1\textwidth}{%
\begin{align*}
\min_{u} \quad & \int_{0}^{t_f} \left[ \frac{u^2(t)}{x(t)} - a u(t) \right] \mathrm{d}t\\
\text{subject to:} \quad & \dot{x} = - u(t) \\
& x(0) = x_0 \\
& 0 \leq x(t) \leq x_0
\end{align*}
}}
\end{document}

%% solution