Find the least common multiple of the numbers by expanding them into primes 21 and 18.

The smallest common multiple of 21 and 18 will be an integer that is evenly divisible by 21 and 18, and will be the smallest of all multiples of them.

To factor a number into prime factors means writing it down as the product of its prime divisors (primes are those numbers that are divisible only by 1 and themselves).

18 = 2 * 3 * 3;
21 = 3 * 7.
To find the LCM, add the factor 7 from the decomposition of 21, which was not in the first decomposition, to the factors of 18. By calculating this product, we determine the LCM:
LCM (14; 18) = 2 * 3 * 3 * 7 = 126.