Answer:
50 degrees
Explanation:
We know that an inscribed angle in a circle is 1/2 the arc that it inscribes. So, therefore the arc is inscribed by the 25 degrees is 50. Assuming that the center of the circle is O, the center angle will be the arc measure. Knowing this, angle a is 50 degrees. If you're curious about all these theorems, they can be proved using similar triangles.
fyi, using the same logic, angle b is 25 degrees