[tex]-2x+7y=18[/tex]
We add [tex]2x[/tex] to both sides:
[tex]2x-2x+7y=18+2x \\ 7y=18+2x[/tex]
We now subtract 18 from both sides
[tex]7y-18=-18+18+2x \\ 7y-18=2x[/tex]
We divide both sides by 2 to leave x at the RHS (Right hand side)
[tex] \frac{7y-18}{2} = \frac{2x}{2} \\ \frac{7y-18}{2} =x[/tex]
Then [tex]x=\frac{7y-18}{2}[/tex]