Diagonal of the cube face, d=√(b2+b2)=b√2
Diagonal of cube l=√(d2+b2)=b√3
Distance between center of cube and vertices, r=l/2=b√3/2
Potential, v=8q/4πϵor=4q/√3πϵob (8 due to presence of 8 charges at the vertices)
by symmetry of charge electric field, E=0.