6 + 4a
is that 3+2a?
6 (x) 4a
is that 6/2 (x) 4/2
The first example you have correct.
In general, (x+y)/z = x/z + y/z, you should know this rule because in grade school you no doubtably spent time finding common denominators in order to add fractions.
The second example, 24a/2 is correct, note that you can reduce that to just 12a.
In the case of multiplication in the numerator, in general xy/z = (x/z)*y = x*(y/z).
In the future if you get confused over these rules, replace your variable with an actual number and see if what you have done is correct.