Let the outer and the inner radii of the sphere be ' R ' cm and ' r ' cm respectively. ∵ Curved surface area of sphere =4π×( radius )2 ⇒4πR2=324π ⇒R2=81 ⇒R=√81=9cm Now, Volume of metal used = Volume of outer sphere - Volume of inner sphere ∵ Volume of sphere =4∕3×π×( radius )3 ⇒684π=4∕3×πR3−4∕3×πr3 ⇒3∕4×684=(9)3−r3 ⇒r3=729−513=216 ⇒r=6cm ∴ Thickness of hollow sphere =R−r=9−6=3cm