A rotation about the line a will produce a cone A whose base radius is 3 inches and height 4 ins.
Rotation about the line b will produce another cone B with radius 4 inches and height 3 inches.
Volume of A will be 1/3 pi 3^3 * 4 while volume of B will be 1/3 pi 4^3 * 3 so B will have the greatest volume because 4^3 * 3 is greater than 3^3 * 4.
Volume of A = 36pi and B is 64pi ins^3
Difference between B and A = 28pi ins^3