A frustum has bottom radius 11 cm, top radius 3 cm and height 6 cm…

A frustum has bottom radius $R$, top radius $r$ and height $h$. Its slant height is $l = \sqrt{h^2 + (R - r)^2}$. The complete cone has height $H = \dfrac{hR}{R - r}$ and slant height $L = \dfrac{lR}{R - r}$. The small cone cut off is similar to the complete cone, so $\dfrac{r}{R} = \dfrac{H - h}{H}$.