line BC is one side of a regular n-gon. The sides next to line BC are extended to meet at W. Find the measure of angle W in terms of n

That's a triangle. Use the facts about the relationships between the angles.

If the ngon is regular, then all sides are equal, and all angles are equal. In this case, n=3

Total sum interior angles = (n-2)180

angle W= totalsum/n