The second column has the sequence 2, 1, 0 from top to bottom. If the pattern keeps up, then the next value in the second column should be -1 since we're counting down by 1. Use a number line to see this.
So we have 2-(-1) = 2+1 = 3 or 2-(-1) = 3 as the next line. I think keeping it as 2-(-1) rather than 2+1 is beneficial so that the minus signs line up.
The result of 3 helps confirm that the final column of values (0,1,2) is incrementing by 1.
