#### Answer

84 $cm^{2}$

#### Work Step by Step

Given sides of a triangle
10 cm, 17 cm, 21 cm
Using Heron's formula if three sides of a triangle have length a,b,c then area A of a triangle is
A = $\sqrt s(s-a)(s-b)(s-c)$
where s = $\frac{a+b+c}{2}$
Lets take a=10 b=17 and c=21
s = $\frac{10+17+21}{2}$
= 24 cm
A = $\sqrt 24(24-10)(24-17)(24-21)$
=$\sqrt 24*14*7*3$
=84 $cm^{2}$