Calculate the molecular weights for nh3 and sf6

no. of moles of SF6 = 0.35/molar mass of SF6 = 0.35/146.06 = 2.396 X 10^-3

so you need 2.396 X 10^-3 moles of NH3 molecules

molar mass of NH3 = 17.031 g/mole

that means 1 mole of NH3 = 17.031 g

so 2.396 X 10^-3 moles of NH3 = 17.031 X 2.396 X 10^-3 = 0.041 g