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We add 2x to both sides:
2x-2x+7y=18+2x \\ 7y=18+2x

We now subtract 18 from both sides
7y-18=-18+18+2x \\ 7y-18=2x

We divide both sides by 2 to leave x at the RHS (Right hand side)
 \frac{7y-18}{2} = \frac{2x}{2}  \\  \frac{7y-18}{2} =x

Then x=\frac{7y-18}{2}