When 215g of potassium nitrate, 0.35g of sodium chloride and 500.0g of water are mixed at about 90'c, 585.3 ml of solution are obtained. What is the density of this solution?

Please show me step by step or I won't understand

You have the mass of all the parts, you also have the volume of the result. Density is mass per unit volume.

Hope that helps.