If ln=54 and lm = 31 then mn = ?

1,643 bc of multiplying or 85 bc of adding but I'm pretty sure it's multiplying

This question is unsolvable,

because there is insufficient information (an angle)

If you have an angle you can use sine rule or cosine rule

sine rule: (sine (distance)) / angle = (sine (distance)) / angle

cosine rule: a² = b² + c² - 2bc cos A