Well a simply way is to look at the problem as
Term 1 | 2 | 3 | 4 | 5 ----------------------------
Value | 9/2 | | | 6
There are 3 terms from 2 to 5 and the value increases 6 - 9/2 = 3/2 so the common difference is 3/2 divided by 3 so the common difference is 1/2 so to find the 3rd term add 1/2 to the 2nd term the pattern is each term is increasing by 1/2.. or 1/2 is added to the previous term.