Explanation
List the multiples of 9 until one is also a multiple of 7. The first six positive multiples of 9 are 9, 18, 27, 36, 45, and 54. None of these are multiples of 7. The next multiple, however, is 63, which is a divisible of 7. Therefore, 63 is a common multiple of 7 and 9. Any other common multiple has to be greater, so 63 must be the least common multiple.