for my c++ class how do you Write a program that check whether a number can be express as sum of two prime numbers. Im supposed to use two functions and call by value

I have code showing me how it is done but I don't understand line 39- 46

Write a program to check whether a number can be express as sum of two prime numbers. - Use two functions - one function is to check the given input is a prime or not. - Both functions will use call-by-value