This could be calculated using the proportion. If the value of the car decreases by 15% each year, that means that after each year the value of the car is 85% of the value from the first year.
Perhaps you mean to find the volume under ? It seems like you have to rely on calculus otherwise (in the case that you indeed mean ).
Assuming this, note that describes the top half of a sphere with radius 8, which means the volume below this surface over the disk is simply half the volume of the sphere. Thus the volume is