You need a common denominator to add/subtract fractions and a form of one to convert them. 24 is common here.
8*2/8*3 = 16/24
- 3*5/3*8 = - 15/24
- 15/24 - 16/24
same as
- 15/24 (+) - 16/24
= - 31/24
======
What number is 2/3 less than -5/8?
x = -5/8 - 2/3 Get LCD of 24
x = -15/24 - 16/24 Subtract
x = -31/24 or -1 7/24 ANSWER