1. The problem is to find the diameter of a circle given its length as 21 cm. 2. The diameter of a circle is the length of a straight line passing through the center and touching two points on the circle. 3. The diameter is given directly as 21 cm in the problem. 4. Therefore, the diameter $d = 21$ cm. 5. No further calculations are needed since the diameter is already provided. Final answer: The diameter of the circle is $21$ cm.