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\begin{document}

\begin{equation}
\label{eq:funkcjaf}
f(x) = \left\{ 
\begin{array}{rcl}
-x^2 & \text{dla} & x \leqslant 0,\\
\sqrt{x} + \sin x & \text{dla} & x > 0.
\end{array}
\right. 
\end{equation}

\end{document}