What have you done? What do you know about circles and triangles?
Given the Base = 2B and the Area = A
1) What is the radius of the semi-circle? B
2) What is the area of the semi circle?
3) What is the area of the triangle? A = ½*B*(height)
4) How do you find the height, given the Base and the Area?
5) How do you find the angle, given the Base and the Height?
You just have to think it through. A solution should present itself.