Hallo runix,
ich habe es in etwa so gemacht:
\documentclass[a4paper,10pt]{scrreprt}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}[2000/07/18]
\usepackage{amssymb}[2002/01/22]
\usepackage[disallowspaces]{mathtools}
\usepackage[detect-all, locale = DE]{siunitx}
\begin{document}
\begin{alignat*}{2}
r^2 &= (\SI{15}{\cm})^2 + a^2\\
r^2 &= (\SI{15}{\cm})^2 + (r - \SI{5}{\cm})^2\\
r^2 &= \SI{225}{\cm^2} + r^2 - r\SI{10}{\cm} + \SI{25}{\cm^2} &&\bigg\vert - r^2\\
0 &= \SI{250}{\cm^2} - r\SI{10}{\cm} &&\bigg\vert + r10\\
r\SI{10}{\cm} &= \SI{250}{\cm} &&\bigg\vert : 10\\
r &= \SI{25}{\cm}\\
A_\text{Halbkreis} &= \dfrac{1}{2}(\pi r^2) &&\bigg\vert (\approx \SI{3,14} * \SI{625}{\cm\squared}) : 2 \\
A_\text{Halbkreis} & \approx\SI{981,75}{\cm^2}
\end{alignat*}
\end{document}
Viel Erfolg
Gruß
Martin
\documentclass[a4paper,10pt]{scrreprt}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}[2000/07/18]
\usepackage{amssymb}[2002/01/22]
\usepackage[disallowspaces]{mathtools}
\usepackage[detect-all, locale = DE]{siunitx}
\begin{document}
\begin{alignat*}{2}
3 \cdot x &= 4 \cdot x + x\bigg\vert -x\\
2 \cdot x&= 4 \bigg\vert : 2\\
x&= 2
\end{alignat*}
\end{document}
