That explains it! 5 years later, and I finally got the answer to this mystery. Now, to actually solve the problem, I still need to know which sides have a length of 7, 10 and sqrt(73) (is it in order: AB, BC, CA?). And M is useless, it should not appear in the problem. Then I can apply proper trigonometry, but for a scalene triangle (not isosceles, not equilateral, not right), that doesn’t look like a high school question. But maybe it depends on countries… or the game takes place in the past, before the math level decreased so much.
Anyway, I came back here because I found this screenshot of my answer that I had apparently never posted (wow it’s small, right-click > Open in new tab to see it better):
