We are given the following system of equations:
[tex]\begin{gathered} x-y=12,\text{ (1)} \\ 2x-y=12,\text{ (2)} \end{gathered}[/tex]The first step to solve the system is to solve for "x" in the first equation, to do that we need to add y to both sides, like this:
[tex]\begin{gathered} x-y+y=12+y \\ x=12+y \end{gathered}[/tex]