How can I find the difference in population by year and zip code using dplyr?

It's hard to help you with your sample data in this format, could you please turn this into a self-contained REPRoducible EXample (reprex)? A reprex makes it much easier for others to understand your issue and figure out how to help.

If you've never heard of a reprex before, you might want to start by reading this FAQ: