In 3(7-4)/2 you have you have to multiply 3 by 7 and by -4 and the answer would be 21-12/2=9/2 or 4.5. While in 3 * 7 -4/2 you will have to multiply 3 by 7 only and your answer will be 21 -4/2=17/2 or 8.5.
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