Calculate charge and current

Homework Statement

The region between concentric conducting spheres is filled with a material with conductivity σ and dielectric constant k. The inner sphere (radius a) is connected to the positive terminal of a battery with emf Vo, and the outer sphere (radius b) is connected to the negative terminal. At t=0 the batter is disconnected.
(a) Calculate Q(t), the charge on the inner conductor, and sketch a plot of this function. Show that the decay constant is kεo/σ.
(b) Calculate I(t).