To display formula inside a box or frame, we must use \boxed function of amsmath package:

\usepackage{amsmath}

For exemple:

$$\boxed{u=1}$$
$$
\begin{equation}
\boxed{
\begin{array}{rcl}
x + -y + z -t & = & 0 \\
2x + 2y + 2z +3t & = & 0
\end{array}
}
\end{equation}
$$