Remember that the formula for pistons is oftena simple rule of three, the input and output force is equal to the division of the force by the area of the application of the force:
Now that is the area of the circle, we just solve the formula of the circle to calculate the radius:
Now remember that the radius is half the diameter so we just have to multiply the radius by 2:
The angle is total is 138 degrees. Part of the entire angle is already given (the 88 degree measure), so all you have to do is subtract 88 from 138, which is 50.