Conceptual understanding of solution / solute in a question
Mole fraction is the ratio of the number of mole of a component to that of the number of moles of the total mixture. In your case, 6.853g of NaCN equals 6.853/49.002 = 0.139 moles.

The rest is water i.e. 107.166 - 6.853 = 100.313g. this corresponds to (100.313/18.015) = 5.568 moles.

Now, total moles = 0.1398 + 5.568 = 5.708moles.

The mole fraction of NaCN = 0.139/5.708 = 0.024.