Now look at the triplet to the left in row 2. Valid combinations for 8 are 125, 134. But there is already a 3 in that block so the only valid combination is 125. So the possible values for the highlighted cell are 1,2 and 5.

Now look at the column triplet containing that cell. Valid combinations for 21 are 489, 579, 678. But there is already an 8 in that column so the only valid combination is 579. This means the cell can only be a 5 or 7 or 9, so we can remove the possibles 1 and 2, leaving 5 as the answer.