-- The weight of the liquid and the flask together is 703.55 g.
-- We know that 345.8 g of that is just the weight of the flask.
So the weight of the liquid alone is (703.55 - 345.8) .
-- The question told us that the volume of the liquid is 225 mL .
-- The density of the liquid is
(its weight)
divided by
(its volume) .
You know the weight and you know the volume, so
you can easily find the density.