The property that justifies the work between steps 7 and 8 is the identity property of addition.
It states the following:
[tex]\begin{gathered} a+0=a \\ 0+a=a \end{gathered}[/tex]
That is, when we have any expression added to zero, the result is the same expression.
So, from step 7 to 8, we can use the property to edit the left side of the equation:
[tex]6x+0=6x[/tex]
