a hollow metal sphere of radius 6cm is charged such that the potential on its surface is 12V.what is the potential at the centre of the sphere?

In the case of a hollow metal sphere (spherical shell), the electric field inside the shell is zero. This means that the potential inside the shell is constant. Therefore the potential at the centre of the sphere is the same as that on its surface, i.e. 12 V. (This is because no work is done in moving a charge inside the shell.)