Remember the definitions of MolaRity and MolaLity. Molarity is number of moles of solute per Liter of solution. Molality is number of moles of solute per kg of solvent. So to figure out the molality of a solution, you need the number of moles of solute and the mass of the solvent.
You're given the mole fraction which states that in one unit of solution, you have 0.15 moles of glucose and 0.85 moles of water. So in one unit of solution, you have 0.15 moles of glucose and 0.85 moles of water. If you know the molar mass of water, you can calculate the mass of water in kilograms present in that one "unit" and then EASILY figure out the molality.