a) Let’s say you have k contacts already. So when x=0 the number of contacts, c, you have is k. That is, c=k.
After a week you c=k+1 people (contacts) altogether. After x weeks you have x new contacts so you have c=k+x people altogether.
b) After 12 weeks you have 12 new contacts so c=k+12 people altogether.
c) It will take you 200-k weeks (x=200-k) for you to have 200 people on your list altogether. That is, 200 new people.