1. The problem asks for the equation for the diameter of a circle. 2. Recall that the diameter of a circle is the length of a straight line passing through the center of the circle and touching two points on its boundary. 3. The diameter is related to the radius by the formula: $$d = 2r$$ where $d$ is the diameter and $r$ is the radius of the circle. 4. This means the diameter is twice the length of the radius. 5. If you know the radius, simply multiply it by 2 to find the diameter. Final answer: $$d = 2r$$