Textbook Problem

In Exercises 15 to 20, use that fact that $r^2+h^2={\mathcal{l}}^2$ in a right circular cone (Theorem 9.3.6).

Find the length of the slant height $\mathcal{l}$ of a right circular cone with $r=6$ in., length of altitude $h,$ $\mathcal{l}=2h$ in.