\documentclass[a4paper,11pt]{article}
\usepackage[polish]{babel}
\usepackage[utf8]{inputenc} 
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{amsmath}



\begin{document}

\begin{align}
\left| z \right| = 0 \iff z = 0,\label{eq:zespolone1}\\
\left| z \right| \geqslant 0,\label{eq:zespolone2}\\
\left| \frac{z_1}{z_2} \right| = 
  \frac{\left| z_1 \right|}{\left| z_2 \right|},\label{eq:zespolone3}
\end{align}

\end{document}