I'm quite confused with the given unit for concentration reported as 'mm'. Usually that would just be 'm' for molality or 'M' for molarity. Based on the given data, I think that would be mM which is 1/1000 of M. So, 170 mM is equal to 0.17 M. With that being said, the solution is as follows:
0.17 M = Moles solute/ 6 L
Moles solute = 1.02 moles
Mass of solute = 1.02 moles * 58.44 g/mole = 59.6088 grams NaCl