Draw the line AB first. Then you get a rectangle ABCD and a segment ABY. Subtract the area of the rectangle and segment from the area of the original cross section (it's basically a circle) to get your answer.

You should know how to find the area of segment (i.e area of sector - area of triangle)

I presume the earlier parts of the question would have given you the radius of the cross section. You will need it to solve these parts.