hello, i have this maths question that i feel like i should be able to do easily but i cant

its arithmetic progressions (again), and the question is to find the sum of the first 8 digits, where the first two are the sum of 18, and the first four are the sum of 52

I'm not asking for all the work to be done, just the method needed so i can follow it and get the answer, which btw is 168 as it says in the back of my book, many thanks