To add these fractions, we first need to find a common denominator. The least common multiple of 24 and 21 is 72.

So, we rewrite the fractions with a common denominator of 72:

- 18/24 = 3/4 * 18/18 = 54/72
- 15/21 = 5/7 * 15/15 = 75/105

Now, we can add the fractions:

54/72 + 75/105 = (54*105 + 75*72) / (72*105)
= (5670 + 5400) / 7560
= 11070 / 7560

Now, we simplify the fraction by dividing both the numerator and denominator by their greatest common divisor, which is 30:

11070 / 7560 = 369 / 252

Therefore, 18/24 + 15/21 = 369/252.