Answer
$$
\left[
\begin{array}{ccc|c}
1 & -1 & 1 & 8 \\
0 & 1 & -12 & -15 \\
0 & 0 & 1 & 1 \\
\end{array}
\right]
$$
Work Step by Step
The coefficients in the first equation are 1,-1,1. These numbers become the entries in the first row of the matrix on the left of the vertical.
The coefficients in the second equation are 0,1,-12. These numbers become the entries in the second row of the matrix on the left of the vertical.
The coefficients in the third equation are 0,0,1. These numbers become the entries in the third row of the matrix on the left of the vertical.
The entries on the right side of the vertical are the numbers on the right side of each of the equations.
$$
\left[
\begin{array}{ccc|c}
1 & -1 & 1 & 8 \\
0 & 1 & -12 & -15 \\
0 & 0 & 1 & 1 \\
\end{array}
\right]
$$
