A petrol tank is a cylinder of base diameter 21 cm and length 18 cm

To find the total capacity of the tank, we have to add the volume of the cylinder and cone.

Diameter of the cylinder, $d=21 \mathrm{~cm}$

Radius of the cylinder, $r=\frac{d}{2}=\frac{21}{2} \mathrm{~cm}$

Height of the cylinder, $h_{1}=18 \mathrm{~cm}$

Also, radius of cone, $r=\frac{21}{2} \mathrm{~cm}$

Also, radius of cone, $r=\frac{21}{2} \mathrm{~cm}$

Height of the cone, $h_{2}=9 \mathrm{~cm}$

Now,

Total capacity of the tank

= Volume of the cylinder + Volume of 2 cones

$=\pi r^{2} h_{1}+2 \times \frac{1}{3} \pi r^{2} h_{2}$

$=\pi r^{2}\left(h_{1}+\frac{2}{3} h_{2}\right)$

$=\frac{22}{7} \times\left(\frac{21}{2}\right)^{2} \times\left(18+\frac{2}{3} \times 9\right)$

$=\frac{22}{7} \times\left(\frac{21}{2}\right)^{2} \times 24$

$=8316 \mathrm{~cm}^{3}$

Hence the total capacity of the tank is $8316 \mathrm{~cm}^{3}$.