The series has two interleaved patterns. The 1st, 3rd, 5th and 7th terms are powers of 3 (3, 9, 27, 81) which is correct. The 2nd, 4th, 6th and 8th terms should each be 4 times the previous even-position term: 7, 7x4=28, 28x4=112, 112x4=448. The 6th term should be 112, but 84 is given, so 84 is the wrong term.