This is something frustrating neither my friends or I understand

I always had problems in finding nth terms and sometimes I do but in paper O/N/07 page 10 question 9 (b) part I couldnt think of a way to find them, neither my friends. Can someone please tell us a solid guide or a way to do it there are 6 questions to find nth term in the Oct/Nov paper year 2007 question 9
We will really appreciate any help, thanks in advance
