For the standard right-opening parabola y2=4ax, a vertical line x=k (where k>0) meets the curve in two points whose ordinates are equal in magnitude and opposite in sign.
a. (i) Derive an expression for the distance between these two points along the y-axis in terms of a and k. Interpret this distance as the width of the parabola at x=k.
b. (ii) Using the expression obtained, compare the widths of the parabolas y2=16x and y2=9x at x=4, stating which one is wider and the exact ratio of their widths.