I don't think you can really simplify it any further... The only thing I can think of is this:
1. Change the first fraction into 18/10m
2. Now change 18/10m into (18/m)/10
3. Change -a/10k into -(a/k)/10
4. Now add them together:
(18/m)-(a/k)
_________
10
It'd be a stretch to call that a simplification. I don't think it is possible.