Summarize the following:
All you have to know is that all of the angles in a triangle always add up to 180°. This is true 100% of the time. So, if you know two of the three measurements of the triangle, then you're only missing one piece of the puzzle. The first thing you can do is add up the angle measurements you know. In this example, the two angle measurements you know are 80° and 65°. Add them up (80° + 65°) to get 145°. The angles in a triangle add up to 180°. Therefore, the remaining angle must make the sum up the angles up to 180°. In this example, 180° - 145° = 35°. You now know that the third angle measures 35°. If you're doubting yourself, just check your work. The three angles should add up to 180° for the triangle to exist. 80° + 65° + 35° = 180°. You're all done.

Summary:
Add up the two known angle measurements. Subtract this number from 180°. Write down your answer.