When simplifying algebraic fractions, do you simplify things attached by addition/subtraction?

Question

EX. 4d-1
_____
6d
Would you be able to further simplify it to:
2d-1
________
3d

Answer 1

no
that would be
2 d - 1/2
----------
3 d

however if the problem had been
4 d - 2
----------
6 d

then you could say
2 d - 1
--------
3d

Answer 2

and if the problem had been:

21a-3
------
7a-6

then it would be:
3a-1
-----
a-2