Short Answer: 150
Remark
You have to use two facts.
1. The central angle = 2 * the angle at B
2. The central angle = the arc angle.
Given
angle at B = 75 degrees.
Calculate the central angle
Central angle = 2 * angle at B
Central angle = 2 * 75
Central angle = 150
Conclusion
The arc angle = Central angle
The arc angle = 150 degrees.
