You are subtracting 3 each time. (-3)
The number to which a base is raised to is called the _exponent_ This tells you how many times to multiply the base together.
7/8+(−2/3) divided by 5/6
(7/8 - 2/3)/(5/6)
=(21/24 - 16/24) / (5/6)
= (5/24) / (5/6)
= 5/24 * 6/5
= 6/24
= 1/4