The height of a cone 21 cm.

It is given that:

Height of the traffic cone (h) = 21 cm

Slant height of the traffic cone (l) = 28 cm

Now we know that,

$\mathrm{I}^{2}=\mathrm{r}^{2}+\mathrm{h}^{2}$

$28^{2}=r^{2}+21^{2}$

$r^{2}=28^{2}-21^{2}$

$r=\sqrt{77} \mathrm{~cm}$

Area of the circular base $=\pi r^{2}$

$=\frac{22}{7} *(7 \sqrt{7})^{2}=1078 \mathrm{~cm}^{2}$

Therefore the area of the base is $1078 \mathrm{~cm}^{2}$