https://www.disk91.com/2015/technology/hardware/design-a-50ohm-impedance-net-for-rf-signals/ ques 2
https://www.chegg.com/homework-help/unilateral-two-port-network-two-port-network-said-unilateral-chapter-17-problem-37p-solution-9781118065587-exc ques 3
https://www.youtube.com/watch?v=pVNwugwMfeM ques 4
Definition for primary and secondary constants of transmission lines:
There are two parameters online constants present in transmission system one is the primary and secondary transmission line constants.
The difference between the two lies in the waveguide present in the secondary and absent in primary.
Primary line constant: Primary line constant is related to simple transmission in which a physical characteristic is related to just one wire maybe copper wire which is responsible for transmission. 
Secondary line constant: Secondary line constant is related to the transmission line but is in contrast with the primary and has a waveguide to low.