\textbf{140-} In solving the system of equations $\begin{cases} x + y - z = 7 \\ 4x - y + 5z = 3 \\ 5x + y + z = 17 \end{cases}$ using Gaussian elimination, the matrix $\begin{bmatrix} 1 & 0 & 0 & a \\ 0 & 1 & 0 & b \\ 0 & 0 & 1 & c \end{bmatrix}$ is obtained. What is $b$?
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(1) $2$ \hfill (2) $3$ \hfill (3) $4$ \hfill (4) $5$
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