List the common multiples of 3 and 7 up to 70.

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

The common multiples of this problem are 21, 42, and 63!