I have a problem I need help with. I have the following Matrix A:

$\displaystyle

\begin{array}{ccccc}

6 & 4 & -1 & 2 & 5\\

3 & 0 & -1 & 2 & 7 \\

18 & 3 & 4 & -2 & 0 \\

6 & 8 & 0 & 0 & -4

\end{array}$

I need to find an elementary matrix with subtracts 2 times column 4 from column 2 of matrix A.

I have no idea where to start, and I can't find a good reference source.