There is an infinite number of common multiples for 9 and 21. A common multiple of any two or more numbers is any number into which each of two or more numbers can be divided evenly (zero remainder).
The common multiples of 9 and 21 are 63, 126, 189 and so on.
Any pair of numbers has an infinite set of common multiples. The lowest common multiple of 9 and 21 is 63 and the others are multiples of this, e.g. 63, 126, 189, 252, 315, ...
The LCM is 63.
The LCM is 63
63.
126
Their lowest common multiple is 63.
63
63, 126, 189
315
If that's 9, 14 and 21, the first three common multiples are 126, 252 and 378.
itzz 63