By dividing the surface into ring elements, find the second moment, about the axis of symmetry, of a conical surface of slant length l and semi-vertical angle 30^o

If the cone is represented by the figure generated when the line y=\frac{\sqrt{3}}{3} x is rotated completely around the x-axis.
The formula for the second moment
\displaystyle M=2\pi\int^a_b y^3 dx
\displaystyle =2\pi\int^{\frac{\sqrt{3}}{2}l}_0 \frac{\sqrt{3}}{9}x^3
\displaystyle = \frac{\sqrt{3}}{32}\pi l^4
Answer is \frac{1}{16}\pi l^4
Thanks!