There are an infinite number of them. The smallest one is 42,
and every multiple of 42 is another one.
I don't remember the official correct way to solve this problem.
I can only show you my own way of doing it.
I take the bigger number, and I go through its multiples, one by one,
until I spot one that's also a multiple of the smaller one.
Example: 6 and 7.
Go through the multiples of 7:
7 ... no, not a multiple of 6
14 ... no, not a multiple of 6
21 ... no, not a multiple of 6
28 ... no, not a multiple of 6
35 ... no, not a multiple of 6
42 ... Yes ! A multiple of 6. yay !