Is it possible to combine process models? If so$,$ provide an example

Suggest the most appropriate generic software process model that might be used as a basis for managing the development of the following systems. Explain your answer according to the type of system being developed:

$?$ A system to control antilock braking in a car

$?$ A university accounting system that replaces an existing system

$?$ An interactive travel planning system that helps users plan journeys with the lowest environmental impact

If you are building a web site selling textbooks, what would be difference in approaches between using iterative $($agile$)$ and waterfall.