Common multiples of 14 and 21 are those and only those integer numbers that are multiple of LCM(14,21) = 42. There are not so many multiples of 42 between 200 and 400. They are 210, 252, 294, 336 and 378.
LCM of 14 and 21 is 42
There are 5 multiples of 42, between 200 and 400. They are: