We can first plug in the values of n, to find a pattern.
When n = 1, 2*1 - 1 = 1
When n = 2, 2*2 - 1 = 3
When n = 3, 2*3 - 1 = 5
...
When n = 15, 2*15 - 1 = 29
We see the pattern, 1, 3, 5, 7, ..., 29.
We find the sum 1 + 3 + 5 + ... + 29.
1 + 3 + ... n + 1 = n ^ 2
(29 - 1)^2 = 784
That is your final answer.
I hope this helped,
Gavin.