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\begin{alignat}{2}
 x &= \underbrace{a + b - a} & &- \underbrace{x + y - x} \\
   &= \phantom{a\:+\:}b      & &- \phantom{x\:+\:} y
\end{alignat}

\[x = \underbrace{a + b - a}_b - \underbrace{x + y - x}_{-y}\]

\documentclass[a4paper,titlepage,11pt]{scrreprt}
\KOMAoptions{captions=tableheading,parskip=full}

\usepackage[fleqn]{amsmath}
\setlength{\mathindent}{0cm}

\begin{document}

\begin{alignat}{2}
x &= \underbrace{a + b - a} & &- \underbrace{x + y - x} \\
&= b & &- y
\end{alignat}

\end{document}