Question 1189060
The volume of the first is pi*8^3=512 π; the second is π*13^3=2197 π units  cm^3.
The total volume is their sum or 2709 π cm^3.
The external radius of the resulting sphere has area 4πr^2 or 900 π cm^2, since r^2=225 cm^2
The thickness is the total volume of 2709π cm^3/900π cm^2
or 3.01 cm.