\( \dfrac{{a - 1}}{2} = \dfrac{{b + 3}}{4} = \dfrac{{c - 5}}{6}\\ \Rightarrow \dfrac{{5a - 5}}{{10}} = \dfrac{{3b + 9}}{{12}} = \dfrac{{4c - 20}}{{24}} = \dfrac{{5a - 5 - \left( {3b + 9} \right) - \left( {4c - 20} \right)}}{{10 - 12 - 24}} = \dfrac{{46 + 6}}{{ - 26}} = - 2\\ \Rightarrow \left\{ \begin{array}{l} \dfrac{{a - 1}}{2} = - 2 \Rightarrow a - 1 = - 4 \Rightarrow a = - 3\\ \dfrac{{b + 3}}{4} = - 2 \Rightarrow b + 3 = - 8 \Rightarrow b = - 11\\ \dfrac{{c - 5}}{6} = - 2 \Rightarrow c - 5 = - 12 \Rightarrow c = - 7 \end{array} \right. \)
