\documentclass[12pt] {article}
\usepackage{pslatex}
\begin{document}
\[
ballistic = 
  \left\{
    \begin{array}{ll}
      \dot{v_x} = 0.0, & v_x(t_0) = v_x,{_0}\\
      \dot{v_y} = -g, & v_y(t_0) = v_y,{_0}\\
      \dot{y} = v_y, & y(t_0) = y_0\\
      \dot{x} = v_x, & x(t_0) = x_0\\
      \\
      v_x,{_0} = cos(theta) \cdot V\\
      v_y,{_0} = sin(theta) \cdot V\\
      theta = 0.7854\\
      V = 20.0\\
      x_0 = 0.0\\
      g = 9.81\\
      y_0 = 2.0\\
    \end{array}
  \right.
\]
\end{document}
