A cube with sides each 3cm long has 8 charges of Q = 25nC at each corner. What is the electric potential \Delta V at the center of the cube with respect to infinity, which is defined to have V=0 V?
Potential is scalar, so figure the potential at the center due to one charge, the multiply it by 8. Scalars add.
distance to charge is tricky. The diagonal in the cube is 3sqrt3, check that, so distance will be half that.