a) To subtract 5/8 from 7/12, we need to find a common denominator. The common denominator for 8 and 12 is 24.

5/8 3/3 = 15/24
7/12 2/2 = 14/24

Now, we can subtract:

15/24 - 14/24 = 1/24

Therefore, 5/8 - 7/12 = 1/24.

b) To calculate 18 * 3 4/9, we first need to convert the mixed number to an improper fraction:

3 4/9 = 3 + 4/9 = 27/9 + 4/9 = 31/9

Now, we can multiply:

18 * 31/9 = 558/9 = 62

Therefore, 18 * 3 4/9 = 62.