By exploiting the above symmetry, or otherwise, calculate the electric potential V at a point on the axis of the annulus a distance d from its center. Be sure to visualize this carefully; the point we are interested in lies along the axis of the annulus, but is above the plane of the annulus by a distance d. Express your answer in terms of some or all of the variables σ, r1, r2, and d. Use k(=14πϵ0).