What is the lowest common multiple of 12 & 21?

84.

Multiples of 12 include: 12, 24, 36, 48, 60, 72, 84, 96, 108, etc…
Multiples of 21 include: 21, 42, 63, 84, 105, etc…
The lowest common multiple between the two is 84. We can check by dividing 84 by both 12 and 21. The quotient in these equations should be factors of the unused multiple. For example:
##84/12=7##
7 is a factor of 21.
##84/21=4##
4 is a factor of 12.