That board and solution remind me a class I had in college on differential equations where after a lot of work from our professor everything started to cancel out and finish with a small elegant solution. After that I asked him how he knows it was a "third degree" equation when there were nothing at the third power on the original problem. He looked at the board and the and the problem and the facepalm and was like "why did you wait until I finished to asking that?" And remade the example in like 3 minutes and 2 lines of solving.
