\documentclass[12pt]{article}
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\begin{document}
\begin{equation*}
  \aligned 
    \dot z &= \dot x - \dot x_d = f(x, u) - f(x_d, u_\text{ff}) \\
           &= f(z + x_d, v + u_\text{ff}) - f(x_d, \underline{u_\text{ff}}) 
           =: F (z, v, x_d(t), u_\text{ff}(t)).
  \endaligned
\end{equation*}
\end{document}