We would say this proposition is about supplementary angles being equal to 180 degrees. Euclid says Proposition 13 is about when “…a straight line set up on a straight line make(s) angles, it will make either two right angles or angles equal to two right angles.”
This artwork inadvertently has many layers, remnants of which are visible through the subsequent layers. That’s a more positive way of saying that the collage and paint I put down didn’t work and I had to cover over them. Several times. I hope the viewer can enjoy the “pentimento” effect!