Lets do this! :D
Alright! So first, you want to focus on that angle on the bottom left on the polygon. Because it is straight angle and you know that the angle adjacent to it is 180-x, that means that the angle of the polygon is x
Now, to solve for x, add all of the angles up in the polygon. Since all quadrilaterals' angles add up to 360, we can make this equation equal to 360. So that gives us:
2x+x+x+18+90=360
Now just simplify
4x+108=360
4x=252
x=252/4
x=63
Therefore, x=63
Hope this helped! :D