I have no paper with me now, but this is what we do.

You see your written numbers 15 and 25 in the right angled triangle? You need to obtain an angle based on it to eventually find angle AOC. The formula for length of arc AC, in radians, is radius of circle x angle AOC.

is the angle 45 degrees?

Let’s call the midpoint of AB as M.

tan AOM = 15/25
AOM = tan-1 3/5 in radians
AOB = pi/2 minus AOM
Then this angle multiply by the radius 25

Typically we do not follow the diagram to scale. Sometimes the drawing is badly done. We use the other given information to deduce the angles.

i see! okay thank you so much