An uncharged, thick spherical conducting shell has an inner radius of \(a = 0.50 \text{ m}\) and an outer radius of \(b = 1.0 \text{ m}\). A point charge \(q = +2.0 \text{ nC}\) is located inside the cavity at a distance \(d = 0.20 \text{ m}\) from the geometric center of the shell. Assuming the electric potential is zero at infinity, what is the electric potential at the geometric center of the shell?