Let us look at the triplet to the right of row 1. We see that the cells must sum to 9. Possible combinations that sum to 9 are: 126, 135, 234. But, because there is already a 3 in row 1 outside the triplet, no cells in the triplet can contain a 3. Therefore the only possible combination is 126. For the highlighted cell in column 9, there is already a 1 and 6 in the column, therefore the cell has to be a 2.