I assume it is millimolar.
So u get millimolar=10-3 molar
1 molar= 1mol/l
1l=1dm3
therefore
a)6 millimolar= 0.006 molar or mol/dm3
b)1 mole= 27X12 + 4 + 6X16=424g
therefore 0.006moles/dm3 = 0.006X424g/dm3
c)conc is 0.006mol/dm3. volume=10cm3 which is 0.01dm3
conc=mole/vol
moles=concXvol
so no of moles of cholestrol in 0.01dm3 are 6X10^-5
hope this helps
therefore mass is 6X10^-5 X 424