how do you convert 7.5% w/v NaCI to mM

I suppose w is weight?

I suspect you mean mass in grams

water is about 1 gram/cm^3
or 1 kilogram/liter
so
our NaCl here is .075*1 = .075 kg per liter or 75 grams/liter

so the real question is
How many mols is 75 grams of NaCl
NaCl = 58.5 grams/mol (23+35.5)
so
75 g (1mol/58.5g) = 1.28 M
(about twice the ocean :)