### Video Transcript

What is the measure of an exterior angle of an equilateral triangle?

The sum of all the exterior angles of a polygon is equal to 360 degrees. An equilateral triangle has three equal sides. Therefore, it has three equal interior angles. This also means that the three exterior angles must also be equal to each other.

We can write this as an equation: 𝑥 plus 𝑥 plus 𝑥 equals 360. Simplifying the left-hand side of the equation gives us three 𝑥 is equal to 360. Dividing both sides of this equation by three gives us a value of 𝑥 equal to 120.

This means that the exterior angle of an equilateral triangle is equal to 120 degrees. We can use this method to calculate the exterior angle of any regular polygon.