1. **State the problem:** We need to find the area of a triangle with base $b = 21$ cm and height $h = 8$ cm. 2. **Formula for the area of a triangle:** $$A_\triangle = \frac{1}{2} \times b \times h$$ This formula states that the area is half the product of the base and the height. 3. **Substitute the given values:** $$A_\triangle = \frac{1}{2} \times 21 \times 8$$ 4. **Calculate the product:** $$A_\triangle = \frac{1}{2} \times 168$$ 5. **Simplify the expression:** $$A_\triangle = \cancel{\frac{1}{2}} \times \cancel{168} = 84$$ 6. **Final answer:** The area of the triangle is $84$ cm².