The sum of the inner angles of any triangle is always 180º
So if for example, you have a triangle with angles ∠1, ∠2 and ∠3 and you add them, the result must be 180º:

This is useful when you know the measure only two inner angles.
To calculate the measure of the third one you just have to subtract the known angles to 180º:

In this exercise, you have the angles
∠1= 23º
∠2= 98º
To determine the measure of the third angle you have to proceed as follows:

The third angle measures 59º