All interior angles of a triangle must have a sum of 180°.
Interior angle of a triangle and its corresponding exterior angle must have a sum of 180° when its line is extended.
Top interior angle is 28°
Lower left exterior angle is 71°
Lower right interior angle is x°
Get the left interior angle.
Exterior angle + Interior angle = 180°
71° + Interior angle = 180°
Interior Angle = 180° - 71°
Interior Angle = 109°
Top Interior angle + Left Interior Angle + Right Interior angle = 180°
28° + 109° + x° = 180°
x = 180° - 28° - 109°
x = 180° - 137°
x = 43°
The lower right interior angle is 43°
