Boxing: \boxed and \Aboxed

In equation, use \boxed{...}:

\begin{equation*}
  \boxed{x} = 1
\end{equation*}

\[\begin{equation*}
  \boxed{x} = 1
\end{equation*}
\]

In align, use \Aboxed{...} from mathtools:

\begin{align*}
  x & = 1 \\
  \Aboxed{y & = 2}
\end{align*}

\[\begin{align*}
  x & = 1 \\
  \Aboxed{y & = 2}
\end{align*}
\]