Simplifying
y + 2x + -1y = 18

Reorder the terms:
2x + y + -1y = 18

Combine like terms: y + -1y = 0
2x + 0 = 18
2x = 18

Solving
2x = 18

Solving for variable 'x'.

Move all terms containing x to the left, all other terms to the right.

Divide each side by '2'.
x = 9

Simplifying
x = 9