### Add multiples of the row containing the red 1 to make the rest of the column zeros.

\(\left[\begin{array}{rrr|r}\large\color{Red} {1} & -3 & -5 & -18 \\
-2 & -4 & 1 & 21 \\
-1 & -1 & -3 & -10\end{array}\right]\begin{array}{ll}\large\phantom{1} & \\
\phantom{1} & \\
\phantom{1} & \end{array}\)