The first multiple is 3*7=21
The further multiples are the multiples of 21:
21, 42, 63
(3*7)
(3*7*2)
(3*7*3)
We can also do this by listing the multiples of each number:
3: 3,6,9,12,15,18,21,24,27,30,33,36,39,42,45,48,51,54,57,60,63,66,69
7: 7,14,21,28,35,42,49,56,63
We can then see that the common multiples are 21, 42, 63
