If the first term, a1 = k. And the nth term an = 2an - 1. To find the fourth term is really simple. The second term of the sequence is a2. So a2 = 2a2 - 1 which is just 2a1. Remember a1 = k. So we have a2 = 2k. Third time, a3, is equal to a3 = 2a3 -1 = 2a2. This gives a3 = 2(2k) = 4k. The fourth term a4 = 2a4 -1 = 2a3. So we have 2(4k) = 8k. Hence the fourth term is 8k.