A sequence is defined by a_1 = 1, a_2 = 3, a_3 = 5, and a_n = a_{n - 1} + a_{n - 2} - a_{n - 3} for n > 3. Find the sum of the first 30 terms of the sequence.

If I understand your question, you have this sequence;

(1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59) =900

Or: Because it has 30 odd consecutive terms, you can also sum it up like this:[30th term]^2 = 900