Which three matrices $E_{21}, E_{31}, E_{32}$ put $A$ into triangular form $U$ ?
$$
A=\left[\begin{array}{rrr}
1 & 1 & 0 \\
4 & 6 & 1 \\
-2 & 2 & 0
\end{array}\right] \quad \text { and } \quad E_{32} E_{31} E_{21} A=U
$$
Multiply those $E$ 's to get one matrix $M$ that does elimination: $M A=U$.