Aball made out of a special material is inflated such that its diameter changes from 14 inches to 18 inches. what is the approximate change in the volume of the ball?

The volume of the ball is:
 V = (4/3) * (pi) * (r ^ 3)
 Where r is the radius.
 We have then:
 diameter 14 inches:
 V1 = (4/3) * (pi) * ((14/2) ^ 3)
 V1 = 1436.75504 in ^ 3
 diameter 18 inches:
 V2 = (4/3) * (pi) * ((18/2) ^ 3)
 V2 = 3053.628059 in ^ 3
 The volume change is:
 V = V2-V1
 V = 3053.628059 - 1436.75504
 V = 1616.873019 in ^ 3
 
 The approximate change in the volume of the ball is:
 V = 1616.9 in ^ 3

Im sorry but what on