Equation numbering inside aligned placed next to an alignedat
Something like this?
\documentclass{scrartcl}
\usepackage{amsmath}
\numberwithin{equation}{section}
\begin{document}
\setcounter{section}{1} % just for this example
\noindent
\begin{minipage}{0.25\textwidth} % choose a suitable width
\setlength\abovedisplayskip{0pt}
\setlength\belowdisplayskip{0pt}
\begin{align}
f_1 &= a + b\\ %1.1 here
f_2 &= a - b %1.2 here
\end{align}
\end{minipage}
\hspace{2cm} % or whatever spacing amount is desired
\begin{minipage}{0.6\textwidth}
$\begin{aligned}
a &= \text{A first line of text}\\
b &= \text{A second line of text}\\
&\quad \text{ And a third line}
\end{aligned}$
\end{minipage}
\end{document}
I don't think this is the best way to set the display.
\documentclass{scrartcl}
\usepackage{amsmath,varwidth}
\usepackage[nopar]{lipsum}
\begin{document}
\lipsum[3]
\begin{center}
\begin{minipage}{0.2\textwidth}% <--- adjust to the needed width
\vspace*{-\baselineskip}
\begin{flalign}
f_1 &= a + b &\\
f_2 &= a - b &
\end{flalign}
\end{minipage}
\qquad
$\begin{aligned}
a &= \text{Some one line text}\\
b &= \begin{varwidth}[t]{0.7\textwidth}
Some two line text\\
second line
\end{varwidth}
\end{aligned}$
\end{center}
\lipsum[4]
\end{document}