We are asked to solve for the angle AOB and we are given with the following values:
radius = 12 cm
length of AB = 15 cm
We can solve for angle AOC such as shown below:
sin AOC = opposite/hypotenuse
sin AOC = 7.5/12
∠AOC =38.68°
The angle subtended at the center of the circle is just twice of ∠AOC, then we have:
∠AOB = 38.68° x 2
∠AOB = 77.36°
The answer is 77° 21' 51.75".