Two alignments within equation

Question

I'd like to write an equation that looks like this:

a = b * c/d
  --> b = a * d/c
        = a * (xyz-zxy)/c

So there should be two alignments. What i've tried so far:

\documentclass{article}
\usepackage[T1]{fontenc}

\usepackage[backend=biber, sorting=none]{biblatex}
\usepackage{amsmath}

\begin{document}

\begin{equation}
 \begin{split}
  a &= b \cdot \frac{c}{d}\\
  &\longrightarrow b = a \cdot \frac{d}{c}\\
  &= a \cdot \frac{xyz-zxy}{c}
 \end{split}
 \label{eq:hauptinduktivität}
\end{equation}\\\\

\end{document}

Answer 1

With minimal editing to your MWE adding in a couple of \hphantoms you can get the = to line up. (You need two as you get different spacing around the = depending on what is either side of it and so its easiest to include = in the \hphantom with and without something next to it.)

\documentclass{article}
\usepackage{amsmath}
\begin{document}
    \begin{equation}
        \begin{split}
            a &= b \cdot \frac{c}{d}\\
            &\longrightarrow\hphantom{=} b = a \cdot \frac{d}{c}\\
            &\hphantom{\longrightarrow b =} = a \cdot \frac{xyz-zxy}{c}
        \end{split}
        \label{eq:hauptinduktivität}
    \end{equation}
\end{document}

Answer 2

Use nested aligned:

\documentclass{article}
\usepackage[T1]{fontenc}

\usepackage{amsmath}

\begin{document}

\begin{equation}
\begin{aligned}
  a &= b \cdot \frac{c}{d}\\
  &\longrightarrow \begin{aligned}[t]
                   b &= a \cdot \frac{d}{c}\\
                     &= a \cdot \frac{xyz-zxy}{c}
                   \end{aligned}
\end{aligned}
\label{eq:hauptinduktivität}
\end{equation}

\end{document}

Never follow \end{equation} with \\. Never use \\\\