1. The problem asks for the exponent used to express the volume of a cube with edge length 21 cm. 2. The formula for the volume $V$ of a cube with edge length $a$ is: $$V = a^3$$ 3. This means the volume is calculated by raising the edge length to the power of 3. 4. Since the edge length is 21 cm, the volume is: $$V = 21^3$$ 5. Therefore, the exponent used to express the volume of a cube is 3. 6. In plain language, the volume of a cube is always the cube (third power) of its edge length.