I can’t recall much of my mathematical education but this looks like trigonometry? My guess is that the builder was working out something to do with the roof. The vertical measurement in 13′ 2″, the length 19′ 5″.
Quite difficult to make out but the first numbers in the equation are those two measurements. Are they trying to work out the length of the other side of the triangle? If so, the builder might be calculating roof measurements. Perhaps someone who understands maths better than me can explain.