Boxed and align

\boxed cannot contain alignment points. Load mathtools (needless to load amsmath in this case) and use \Aboxed:

\documentclass{article}
\usepackage[x11names, rgb]{xcolor}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}

\begin{document}

\begin{align}
  \Aboxed{ 2x+y & = 11 } \\
  3x & = 19 - 2y
\end{align}

\end{document} 

You can use tikz to create frame

\documentclass{article}
\usepackage[x11names, rgb]{xcolor}
\usepackage{tikz}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}

\def\tikzmark#1{\tikz[remember picture,overlay]\node[inner ysep=0pt,anchor=base](#1){\strut};}

\begin{document}

\begin{align}
  \tikzmark{A}2x+y & = 11 \tikzmark{B} \\
                3x & = 19 - 2y
\end{align}

\tikz[remember picture,overlay]\draw(A.south west)rectangle(B.north east);

\end{document} 

Here's a workaround that works with MathJax, but it's not pretty and might need additional tweaking depending on the MathJax implementation you're working with:

$$ 
\begin{align*}
2x+y &= 11 \\ 
3x &= 19 - 2y 
\; \llap{\mathrel{\boxed{\phantom{3x = 19 - 2y}}}}
\end{align*} 
$$

Here's what this would look like on math.stackexchange.com:

Edit: I boxed the wrong line by mistake, but one can accomplish the same thing for any line within an align block.