Three concentric uniformly charged isolating shells (hollow spheres) of radii 6cm, 10cm, and 20cm each carry a charge. The smallest shell carries 3nC, the medium 4nC, and the largest shell 5nC. Here, concentric means that all three shells share the same center point. Calculate the magnitude of the electric field at a point 15cm from the center point along the x-axis, indicate the direction of the electric field in your diagram.