1. In the 1st row you can see that the number in 2nd column (84) is reversed (48) and added to the number in the first column (51) to obtain the number in the 3rd column (99).

2. Similary, in the 3rd row the number in 2nd column (12) is reversed (21) and added to the number in the first column (13) to obtain the number in the 3rd column (34).

3. Hence in the 2nd row, you need to add 47 to the number in 1st column 8 to obtain the number in 3rd column (55).