How to align the equation below with the ones above

I would use 1 outer and 2 inner array environments, along with the midrule macro of the booktabs package.

documentclass{article}
usepackage{array}    % for 'newcolumntype' macro
usepackage{booktabs} % for 'midrule' macro
newcolumntype{C}{>{{}}c<{{}}} 
newcommandxx{hphantom{0}} % handy shortcut macro

begin{document}
[
setlength{arraycolsep}{0pt}
begin{array}{r} % begin of 'outer' array
left{
begin{array}{rCrCrCr} % 1st 'inner' array
2x &+& 3y &+& 4z &=& 7 
 x &-& 7y &+&  z &=& 8 
4x &-& 5y &+&  z &=& 9 
end{array} 
right.kern-nulldelimiterspace  
   % 'kern-nulldelimiterspace' gobbles up the space that's inserted by 'right.'
midrule
begin{array}{rCrCrCr} % 2nd 'inner' array
 xx x &+& xx y &+& xx z &=& 3 
end{array}
end{array} % end of outer array
]
end{document}