A volte le cose sono più semplici di quanto si pensi.:D 😀
Direi che così ti puoi accontentare senza porsi troppi problemi (non c’è nemmeno bisogno del pacchetto mathtools):
`
\begin{gather}
\makebox[.8\linewidth]{\hfill$\forall t \in \mathopen{(}0,T\mathclose{]} \quad \mathrm{trovare} \quad
(\stackrel{\circ}{\Ub}\!\!(t),P(t)) \in \Vb_\zerob \times Q \,
:$}\nonumber\\[2mm]
\begin{cases}
\dsfrac{d}{dt}(\rho\stackrel{\circ}{\Ub}\!\!(t),\vb)+c^{\rho}(\stackrel{\circ}{\Ub}\!\!(t),
\stackrel{\circ}{\Ub}\!\!(t),\vb)+c^{\rho}(\stackrel{\circ}{\Ub}\!\!(t),\Rb_{\Ub_D},\vb)\\[2mm]
\qquad\qquad+c^{\rho}(\Rb_{\Ub_D},\stackrel{\circ}{\Ub}\!\!(t),\vb)+\dsfrac{1}{\mathrm{Re}}a^{\mu}
(\stackrel{\circ}{\Ub}\!\!(t),\vb)+b(\vb,P(t))\\[4mm]
\hspace{4cm}=\mathrm{G}\mathcal{F}^{\rho}(\vb)+\mathcal{F}^{R_1}(\vb)\quad \forall \vb \in \Vb_\zerob,\\[2mm]
b(\stackrel{\circ}{\Ub}\!\!(t),q)=\mathcal{F}^{R_2}(q)\qquad\forall q\in Q,\\[2mm]
\stackrel{\circ}{\Ub}\!\!(0)=\Ub_0,
\end{cases}
\end{gather}
\end{gather}
`
Ciao
Andrea