\documentclass[a4paper,11pt]{article}
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\begin{document}

\begin{equation}
\label{eq:macierz:uzupelniona}
U = \left[ \begin{array}{cc} A & b \end{array} \right] =
\left[ 
\begin{array}{lllll}
a_{11} & a_{12} & \dots & a_{1n} & b_1\\
a_{21} & a_{22} & \dots & a_{2n} & b_2\\
\vdots & \vdots & \ddots & \vdots & \vdots\\
a_{m1} & a_{m2} & \dots & a_{mn} & b_m\\
\end{array}
\right]
\end{equation}

\end{document}