They are: 63, 126.
The first two multiples are 9 and 8.
63, 126 and 189
63, 126, 189
The common multiples of 9 and 21 are 63, 126, 189 and so on.
If that's 9, 14 and 21, the first three common multiples are 126, 252 and 378.
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).
lcm(4, 9) = 36 → the two least common multiples (ie the first two common multiples) of 4 and 9 are 36 and 72.
126, 252, 378
63, 126, 189, 252, 315 . . .
63, 126, 189
Common multiples are multiples that are common to two or more numbers.9: 9, 18, 27, 36, 45, 54, 63, 72, 81, 90, 99, 108, 117, 126, 135, 144, 153, 162, 171, 180, 18921: 21, 42, 63, 84, 105, 126, 147, 168, 189The first 3 common multiples of 9 and 21 are 63, 126 and 189.63, 126, 189.
LCM(9, 21) = 63 so the numbers are 63, 63*2 and 63*3.