Okay, here we have this:
Considering the provided graph, we are going to calculate the requested measure of the angle, so we obtain the following:
So let's remember Thales' theorem, which tells us that if A, B, and C are distinct points on a circle where the line AC is a diameter, the angle ABC is a right angle.
Therefore, in this case, since the triangle has a straight line that corresponds to the diameter of the circle, we can state with total certainty that angle "A" is a right angle, that is, it measures 90°.