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



\begin{document}

\begin{equation}
\label{eq:uklad:rownan}
\left\{
\begin{array}{l}
x + y = 8\\
3x - y = 0
\end{array}
\right.
\end{equation}

\end{document}