\begin{array}{l} \text { a. } \quad \begin{array}{l} 2 x_{1}-x_{2}+5 x_{3}=3 \\ x_{1}-8 x_{2}+2 x_{3}=5 \\ 4 x_{2}-4 x_{3}=5 \end{array} \end{array} \begin{array}{ll} \text { b. } & x_{1}+6 x_{2}+2 x_{3}-x_{4}=5 \\ & 5 x_{1}-6 x_{3}=7 \end{array} \begin{array}{l} \quad x_{1}+5 x_{2}=-3 \\ -2 x_{1}-13 x_{2}=8 \\ 3 x_{1}-3 x_{2}=1 \end{array} 19. For each of the following systems of linear equations, write down the equivalent linear vector equation:

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