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\begin{document}

\begin{equation}
\label{eq:calka}
\begin{split}
\int x^2 e^x dx & = x^2 e^x - 2 \int x e^x dx = \\
& = x^2 e^x - 2 \left( x e^x - \int e^x dx \right ) = \\
& = x^2 e^x -2x e^x + 2 e^x + C
\end{split}
\end{equation}

\end{document}