Question

find the missing numbers 3,__,__,12__,__,21

Answer 1

multiples of 3
3 6 9 12 15 18 21

Answer 2

theres a differnece of 9 between 12 and 3  and also between  12 and 21

looks like an arithmetic series with common difference = 3

so the ful series is 
3,6,9,12,15,18,21