Final answer:
The 'fixed length' for the '3.50 m side' would simply be 3.50 m, which is an option (c), as it matches the side mentioned. Without more context, a detailed explanation cannot be provided.
Step-by-step explanation:
It appears there might be some confusion in the details of the question provided. However, if the goal is to determine the fixed length corresponding to the '3.50 m side', with the options provided, the answer would be (c) 3.50 m because it directly specifies the 3.50 m side. Without additional context, it is not possible to provide a step-by-step explanation. If the question relates to a geometric problem such as calculating the length of a side in a polygon given other sides, or an optics problem such as finding the image distance in a lens equation, further information would be required to solve the problem. Relevant examples can't be provided without additional information about the nature of the 'fixed length' in question.