To find the length of \( CD \), we can use the information provided. However, you haven't specified what \( C \) and \( D \) represent in this context. If \( C \) and \( D \) are coordinates, for example, the length \( CD \) could be calculated using the distance formula, depending on their respective coordinates.
Given \( C = 2 \) and \( D = 20 \), if these are points on a straight line:
\[ CD = |D - C| = |20 - 2| = 18 \]
So, the length of \( CD \) is 18. If you meant something else, please clarify!