The concentration given here is in terms of molality which is moles of solute per kg of solution. Therefore, we multiply the concentration by the total mass of the solution and obtain the moles of solute. We can convert this into grams and calculate for the solvent.
0.630 mol KBr / kg solution ( .086 kg solution ) = 0.0542 mol KBr
0.0542 mol Kbr ( 119.002 g / 1 mol ) = 6.4475 g KBr
