Solving system of equations by echelon form. I have an example of a solution for a system of equations and there is a step I do not understand: 
Could anyone offer some help please?
Kind regards
Stany

$$
\begin{aligned}
\left(\begin{array}{cccc}
1 & 1 & 1 & 0 \\
3 & -1 & 1 & 20 \\
6 & 2 & -1 & 40
\end{array}\right) & \rightarrow\left(\begin{array}{cccc}
1 & 1 & 1 & 0 \\
0 & -4 & -2 & 20 \\
0 & -4 & -7 & 4
\end{array}\right) \\
& \rightarrow\left(\begin{array}{cccc}
1 & 1 & 1 & 0 \\
0 & -4 & -2 & 20 \\
0 & 0 & -5 & 20
\end{array}\right) \\
\therefore-5 z=20 & \Rightarrow z=-4 \\
-4 y-2 x(-4)=20 & \Rightarrow y=-3 \\
x-3-4=0 & \Rightarrow x=7
\end{aligned}
$$
Hence $(x,y,z)=(7,-3,-4).$
