Please See Attached
You are going to be using the formula
First see that you are taking the integral from until that is when it intersects the x and y axes.
Now you need to find :
Notice that implies .
At the width of the line perpendicular to the x-axis is then . But this is the diameter of the circle. Thus, its radius is .
The area of this cross section (semicircle) is
(remember the 1/2 it is semi-circle )
I think there is something wrong with your choices.