The measure of the vertex angle of an isosceles triangle is 120. what is the measure of the base angle?

We know that the sum of interior angles in the triangle is 180°.
2 α + β = 180°
The vertex angle is β = 120°
2 α + 120° = 180°
2 α = 180° - 120°
2 α = 60°
α = 60° : 2 = 30°

The base angle is : α = 30°