If you wanted to have a solution of benzene that would freeze at 0 degrees Celsius with NaCl, what would be the mole fraction of NaCl that you would need?
What's the freezing point of benzene?
delta T = i*Kf*molality
i = 2 for NaCl.
Kf = ?? for benzene.
solve for molality.
Then molality = moles/kg solvent.
Convert 1 kg solvent (benzene) to moles. moles = grams/molar mass.
Finally, mole fraction NaCl = moles NaCl/(moles NaCl + moles benzene)
Post your work if you get stuck.