By row:
8 x 3 = 24. Divide by 6 = 4
9 x 4 = 36. Divide by 6 = 6
12 x 2 = 24. Divide by 6 = 4By column:
8 x 9 = 72. Divide by 6 = 12
3 x 4 = 12. Divide by 6 = 2
4 x 6 = 24. Divide by 6 = 4

I agree that if you work in columns then by adding the first two numbers and subtracting 5 then you get an answer of 5 as explained by various people above. However, if you work by row and add the first two numbers then subtracting 5 won’t work [you need to subtract 7] to get the final number.

Multiplying the first two numbers and then dividing by 6 works horizontally and vertically.