Answer:
The Triangle Sum Theorem is also called the Triangle Angle Sum Theorem or Angle Sum Theorem. It says that all the angles inside a triangle add up to 180°.
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Take a look at the picture attached. It has a "formula": ∠a+∠b+∠c=180°
Use this as it will help you a lot. You plug in the info you already know (it doesn't matter which angle is a,b,c) and solve for the missing one. :)
Ask in the comments below if you are not sure about something.
I hope that helps you! :D