How to understand Transmission and Propagation delay formula
For a:

The term over is a common term to express a communication channel - we send data over a link.

Each link (a and b) are set to m metres. Therefore the total distance the signal must travel from host A to host C is m + m metres; i.e. 2m metres.

For b:

I am assuming something here; the packet from host A must be completely received by host B prior to host B then forwarding the packet to host C although it is not explicitly stated. The transmission delay is therefore the complete transmission delay from host A to host B and then the packet must be placed on the link to host C and incurs another transmission delay of L bits.

That being the case, the transmission delay is the cost of placing the packet on the link twice, yielding 2L bits of delay.

For c:

This is simply looking for the case of where the transmission delay is equal to the propagation delay of the link. This requires a solution in terms of distance (which is the only contributor to propagation delay for this question).

The solution is a relatively simple algebraic re-arrangement and finally the numbers are plugged in (multiply both sides by s and then divide both sides by 2).