Ok so first you know that the missing angle is going to = y+x. Then look at just half to the x so you’re looking at only a “straight line”. That means you are either seeing y+x and 2x together or y-x and y+x together. Either way, both add to 180, so that means they are equal to each other! So, you can set them equal to each other: 2x + y + x = y+ x + y - x. When you simplify that, you get y+3x = 2y so 3x = y. Then you can go back to your first equations, I used 2x + y+x = 180 and where ever there’s a y substitute in 3x. So you are left with 3x + x + 2x = 180 —> 6x = 180 so x = 30°!