It looks like you wrote the equation in point-slope form perfectly!
But for part c, it seems as though x1 and y1 would represent parts of the midpoint formula. So, I would specify and say that they represent the midpoint, not just a point.
The two curves intersect at two points, (0, 0) and (8, 8):
The area of a semicircle with diameter is . The diameter of each cross-section is determined by the vertical distance between the two curves for any given value of between 0 and 8. Over this interval, and , so the volume of this solid is given by the integral