#### Answer

84 $cm^{2}$

#### Work Step by Step

The area of triangle whose base has length b and whose corresponding altitude has length h is given by A = $\frac{1}{2}$ bh
In triangle ABC base b= 21 cm
Height h = 8 cm.
A =$\frac{1}{2}$ bh
= $\frac{1}{2}$* 21 * 8
= 84 $cm^{2}$