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

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

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