13 is the 7th term and 39 is the 20th term
Now we have sum of n terms is n^2. So take a difference of the squares of poisitions of 39 and 13 which is 400-49 = 351
gmat2me2, you have right approach. but you overlooked and miscalculated. it says including 13 and 39. it should be =20^2-6^2=364.
or alternatively, sum = n(F+L)/2 = 14(13+39)/2 = 14x26=364