#how to make a brace below a text ?

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limpid mango
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here is my code

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\item[\textbf{2.}] On injecte dans (E) : pour tout $t \in I$,
    \begin{center}
    \begin{tabular}{r c l l}
    $\varphi (t)$ & = &$ \lambda \times e^{-A(t)}$ & \vline $\quad \times  a(t)$ \\
    $\varphi '(t)$ & = &$ \lambda '(t) \times e^{-A(t)} - \lambda(t) \times a(t) \times e^{-A(t)}$ & \vline $ \quad \times 1$ \\
    \hline
    $\begin{array}{@{}c@{}} \varphi '(t) + a(t) \times \varphi (t) \\ \vspace*{-8pt} \underbrace{\phantom{\varphi '(t) + a(t) \times \varphi (t)}}_{\text{b(t)}} \end{array}$ & = &$ \lambda '(t) \times e^{-A(t)}$ & \vline \\
    \end{tabular}
    \end{center}