I think the volume of the cone is 65.94yd because the radius is 3 and the height is 7 also 3 to the second power equals 9 then 3.14 multiplied by 9 equals 28.26 and finally 28.26 multiplied by 7/3 equals 65.94
a is the beggining number and n is the amount of times needed to do this. if is 17 you would need to subtract 7 from the originol number 12 (a) that many times.