Berechnen Sie den Wert der Variablen u, sofern eine Lösung existiert.
a) u \( \cdot\left(\begin{array}{l}3 \\ 5 \\ 1\end{array}\right)=\left(\begin{array}{l}1 \\ 2 \\ 1\end{array}\right)-\left(\begin{array}{c}7 \\ 12 \\ -1\end{array}\right) \)
b) \( \left(\begin{array}{l}4 \\ \mathrm{u} \\ 2\end{array}\right)+2\left(\begin{array}{l}1 \\ 2 \\ 3\end{array}\right)=\left(\begin{array}{c}\mathrm{u} \\ 10 \\ \mathrm{u}+2\end{array}\right) \)
c) \( \left(\begin{array}{r}20 \\ 4 \\ -14\end{array}\right)=u \cdot\left(\begin{array}{c}12 \\ 4 \\ 4\end{array}\right)-2 u \cdot\left(\begin{array}{l}1 \\ 1 \\ 3\end{array}\right) \)