Hello Dear,

1. One charge Q is present on the thin spherical shell , so the shell is totally charged Q.
2. Second charge Q is placed at the center of the spherical shell.
3. Now for potential at point P we have

• Let V1 is potential at P due to Q on spherical shell and let V2 is potential at P due to center charge, let V is total potential at P. So now

V=V1+V2

V=0 + 9*10^9*Q*2/d

V=18*10^9*Q/d