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\begin{equation*}
  (m_1+m_2) \frac{d^2z_1}{dt^2} + c_2\frac{dz_1}{dt} + k_2 z_1 
  = m_2 \frac{d^2l}{dt^2} + c_2 \frac{dl}{dt} + k_2 l.
\end{equation*}
\end{document}