155015

We added up all the values from 7 to 15, which gave us 99 and them divided by three because there are three rows. This means each row/ column/ diagonal adds up to 33. Since we know 7 and 14 are in a row, 33- (7+14) = 12. We substituted 11 and 15 into the middle row as it needed two numbers which add up 26. We worked out that 11 must go in the middle as otherwise the diagonal would need a 6 in the bottom left hand corner and 6 isn't one of the options. We added in all the other numbers to check that it would work, which it did and then we knew that n must be 8