molar mass of solute is 30 g/mol
molality is defined as the number of moles of solute in 1 kg of solvent
number of moles of solute - 5.0 g / 30 g/mol = 0.17 mol
molality of solution is 0.70 mol/kg
the mass of solvent when there are 0.7 mol of solute - 1 kg
then the mass of solvent when there's 0.17 mol of solute - 0.17 mol / 0.7 mol/kg
= 0.242 kg
therefore mass of solvent is 0.242 kg
