The augmented matrix for the given linear system and its row-reduced form are:
\begin{align*}
\begin{bmatrix}
1 & 2 & 3 & 0\\
2 & -1 & 1 & 2\\
1 & -8 & -7 & 1\\
0 & 1 & 1 & 0
\end{bmatrix}
\rref
\begin{bmatrix}
\leading{1} & 0 & 1 & 0\\
0 & \leading{1} & 1 & 0\\
0 & 0 & 0 & \leading{1}\\
0 & 0 & 0 & 0
\end{bmatrix}.
\end{align*}
For this system, we have $n = 3$ and $r = 3$. However, with a pivot column in the last column we see that the original system has no solution by Theorem RCLS.