Uva 10059 link

I was trying to solve this problem but couldn't come up with anything except the intersection point of the given lines. I would appreciate any ideas on solving/approaching this problem . Besides, what topics should I learn to solve it ?

Problem Link : https://www.spoj.com/problems/PRIMES2/ My Tle Solution : https://pastebin.com/wXtp0wVf In this problem you have to generate primes upto 10^9 . Time limit: 2.281s

Link: https://pastebin.com/FRTreAiJ

I come across this type of implementation for fast input of integers in geeksforgeeks.com site . Can this code be improved to input integers faster ? ( getchar_unlocked() doesn't work in Windows . )

What are some efficient ways to divide 'a' by 'b' where the range is :

-10^300 <= a,b <= +10^300