Explanation:

To find the area of a triangle with sides 21 cm, 20 cm, and 29 cm, use Heron's formula:

- Calculate the semi-perimeter (s): $s=\frac{21+20+29}{2}=35$.

- Use Heron’s formula for the area (A):

$A=\sqrt{s\cdot (s-a)\cdot (s-b)\cdot (s-c)}$.

So,

$A=\sqrt{35\cdot (35-21)\cdot (35-20)\cdot (35-29)}$.

Simplified,

$A=\sqrt{35\cdot 14\cdot 15\cdot 6}$.

- Calculate:

$A=\sqrt{44100}$, which results in 210 cm².

- 210 CM² is the correct answer.