Why can we consider the prime ideals of $\Bbb Z$ to determine the prime ideals of $\Bbb Z[x]$?
There really isn't any work I can show here. Motivation is that it seems various papers consider a prime ideal of $\Bbb Z$ and then consider cases to single out what this prime ideal of $\Bbb Z$ looks like and then they say these are the prime ideals of $\Bbb Z[x]$