Hello kevin3000You say that you can see how . You are right, and the reason that it's equal to the marked angle of is that they are both subtended by the arc .
We say that angles like this are Angles in the Same Segment - and they're equal.
(When you divide a circle into two by a chord you get two segments - the bigger one is the major segment, and the smaller one is the minor segment. Here the chord is , and the angle and are both in its major segment.)
Well now, can you spot the arc that is subtending the marked angle? It is the arc . And this arc also subtends . So these angles are also equal - they're in the major segment of the chord . So .
I'm sure you know how to find the third angle of the triangle, , so I'll leave it up to you.
Grandad