documentclass{article}
usepackage{subfigure,graphicx,amsmath,amssymb, amsfonts,physics,empheq}


begin{document}


    begin{empheq}[left=empheqlbrace]{align}
    Q_2(s) &= frac{k_{21} Q_1(s)}{s+k_{12} + k_{02}} \
    Q_3(s) &= frac{k_{31} Q_1(s)}{s+k_{13} + k_{03}} \
    s , Q_1(s) + (k_{21} + k_{01}) Q_1(s) - frac{k_{12}k_{21} Q_1(s)}{s+k_{12} + k_{02}} - frac{k_{13}k_{31} Q_1(s)}{s+k_{13} + k_{03}} &= U_1(s) \
    Y_1(s) &= frac{Q_1(s)}{V_1} \
    H(s) &= frac{Y_1(s)}{U_1(s)}
    end{empheq}



end{document}