A multiple is any number that can be divided by another number to result a number. So, for multiples of 18, you are looking for number that can be divided cleanly and result in 18. To find multiples easily, multiply 18 by numbers.
18 * 1 = 18
18 * 2 = 36
18 * 3 = 54
18 * 4 = 72
18, 36, 54, and 72 are a few of 18's multiples.
Note multiples can also be negative.
18 * -1 = -18
18 * -2 = -36
18 * -3 = -54
18 * -4 = -72
-18, -36, -54, and -72 are also multiples of 18. I hope this helps! @-Happy7