Table attached is for reference:

To simplify our analysis, let's assume:

The network topology above $($i$.$e$.$ two hosts connected by two routers and three links$)$ is used. The transmission rate of each link is bps$.$ Size of the original message that the "Source" node sends to the "Destination" node is bits. The "Source" node conducts message segmentation. The size of each segment is bits. The "Source" node adds $20$ bits of header fields to each packet it sends.

Derive a mathematical formula for D as a function of F$,$ S$,$ and R$.$

Then, to calculate the value of S which leads to the minimum D $,$ find out the derivative of D with respect to S and set it to $0.$ Ensure to highlight your final answer. Verify your answer by comparing it with pertaining values in the table attached$($i$.$e$.$ that last six entries where size of headers is $20$ bits$)$