The fractional part of the first number is too small so let's borrow one from it:
8 and 4/6 can be written as 7 and 6/6 and 4/6.
I took a full unit from the 8 and wrote it in terms of sixths.
Now combine the fractional parts,
7 and 10/6,
and we're taking away 2 and 5/6 from this amount.
7 - 2 = 5
10/6 - 5/6 = 5/6
giving us 5 and 5/6 as a result.
Understand? :)
