For the six numbers, the problem gives (8+9+16 + (sum of other three))/6 = 12
8+9+16= 33 and call the Sum of the other three numbers S.
33 + S = 72
S= 72-33=39
Apply the average formula. S is the sum of the three numbers, and there are 3. S/3 = average. 39/3 = 13 is the average of the other three numbers.