The angle sum of a triangle is always 1800.
When dealing with angles of triangles in geometry, it’s always the angles inside the triangle that are being used, not the angles outside.
When two angles of a triangle are given, and the third is missing, you can always calculate the third with this simple formula:
If only one angle but more than one of the side lengths are given, it’s possible to calculate the missing angles by use of trigonometry. More about that further on.