You have the following fractions:
8/9 + 3/8
In order to fins the least common denominator, you find the least common multiple of the denominators. To do that, list the multiples oh both number until the same number appears in both lists, just as follow:
for 9:
9, 18, 27, 36, 45, 54, 63, 72
for 8:
8, 16, 24, 32, 40, 48, 56, 64, 72
Hence, the least common denominator is 72.