Well, let's break this down, line by line:
3 int variables are declared: i, j, and a. a is set to 0.
a FOR loop is entered: i = 0; i < 3; i++
a FOR loop is entered: j = 0; j < 2; j++
And in each execution of the inner loop, 2 * i + j - 1 is added to a.
The outer loop executes three times, and for each execution of the outer loop, the inner loop executes twice. SO the total number of operations are 2 * 3 or 6. These operations happen at the (i, j) pairs:
(0, 0)
(0, 1)
(1, 0)
(1, 1)
(2, 0)
(2, 1)
That is, the previous 6 combinations are the values of i and j, respectively, in each addition. So, with a = 0 at the start,
a = a + (2 * i + j - 1)
a = 0 + (2 * 0 + 0 - 1)
a = 0 + (0 - 1)
a = -1
After the first execution, a is -1. Then the next addition comes:
a = a + (2 * i + j - 1)
a = -1 + (2 * 0 + 1 - 1)
a = -1 + (0 + 0)
a = -1
After the second execution, a is still -1.
Continue doing this step by step for the last 4 (i, j) pairs to see what a ends up as.