To add these fractions, we need to find a common denominator. The least common multiple of 18 and 21 is 126.

Now, we can rewrite the fractions with the common denominator:

12/18 = (12 * 7)/(18 * 7) = 84/126
15/21 = (15 * 6)/(21 * 6) = 90/126

Now, we can add the fractions:

84/126 + 90/126 = (84 + 90)/126 = 174/126

The fraction 174/126 cannot be simplified further, so the final answer is 174/126.