y = 2x-3
x + y = 18
Since you already know what y is equal to, we can plug that in into the second equation.
x + (2x-3) = 18
x + 2x -3 = 18
3x -3 = 18
Add 3 to both sides of the equation. (By doing this, we can isolate the 3x on the left side by getting rid of the -3)
3x = 21
Divide 3 on both sides
x = 7