That's worded terribly. Basically it's saying that if you're rotating the area about a point that is inside/on the edge of the area, you'll get slices that are disks. Whereas if it's outside the area you'll get a slice that's an annulus because there's a gap between the area and the axis of rotation. Even that's not worded great - it's much easier to understand if you look at it graphically.
I can't be bothered actually doing it, but I'll tell you how. Solve them simultaneously and you'll get a quadratic in x. Take half the sum of roots (i.e. -b/2a) and that's the x-coordinate of the midpoint. Substitute that into the equation for PQ and you'll get the y-coordinate.