In a circle, inscribed angles that subtend the same arc are equal. Additionally, an inscribed angle is half the measure of the arc it intercepts.
In triangle LMN, since angle N is the inscribed angle, it intercepts the arc MN. We know that the diameter LM subtends a semicircle, which means that the arc LM measures 180°.
The angle at vertex L is given as 37°. This angle subtends arc MN (the arc opposite angle L). According to the inscribed angle theorem, we can find the measure of arc MN:
\[ \text{measure of arc MN} = 2 \times \text{measure of angle L} \]
Substituting the value of angle L:
\[ \text{measure of arc MN} = 2 \times 37° = 74° \]
Therefore, the measure of arc MN is 74°.