Idk I just needed to ask the question I don't even know what this means

The "multiples" of a number, N, are the numbers N, 2*N, 3*N, ..., etc. For example, the multiples of 4 are listed below in the usual order.

4, 8, 12, 16, 20, ..., and so on.

So by "common multiple" one means a number that is a multiple of 2 or more numbers.

Try listing the multiples of 7 and 21, separately.

You will find many numbers that appear in both lists. These are the common multiples of 7 and 21.

In fact, you will find that every multiple of 21 is also a multiple of 7. Do you see why?

Hope this helps. If not, I or another tutor will be glad to further assist you.