Ok so the volume of a sphere is V = (4[tex] \pi [/tex] r^3) / 3
So in the 3cm sphere the volume is V = 4 [tex] \pi [/tex] 27 / 3 --> V = 36 [tex] \pi [/tex]
and in the 6cm sphere: V = 4 [tex] \pi [/tex] 216 / 3 --> V = 288 [tex] \pi [/tex]
So it's 8 times bigger! (288 / 36 = 8)