[tex]\bf \textit{surface area of a cone}\\\\ SA=\pi rs+\pi r^2~~ \begin{cases} r=&radius\\ s=&slant\\ &height\\ \cline{1-2} SA=&16.8\pi \\ r=&3 \end{cases}\implies 16.8\pi =\pi (3)s+\pi (3)^2 \\\\\\ 16.8\pi =3\pi s+9\pi\implies 16.8\pi -9\pi =3\pi s\implies 7.8\pi =3\pi s \\\\\\ \cfrac{7.8\pi }{3\pi }=s\implies 2.6=s[/tex]
