The density of marble is between 2.6 and 2.8 grams per cm³ .
Density doesn't depend on how much mass or volume of it you have.
The density of a chip of it is the same as the density of a truckload of it.
You need the solubility of the LiCl in water at 20°C.
The solubilities are shown in tables. Many books contain those tables. You have to make sure that the solubility is shown in the same solvent and at the same temperature that you are going to prepare the solution.
In this case the solubility of LiCl in water at 20°C is 83.05 g of LiCl in 100 g of H2O. Check if your book and your teacher work with the same value.
Using 8.05 g LiCl / 100 g of water you get:
300 g of water * 83.05 g LiCl / 100 g of water = 249.15 g of LiCl.
Answer: 249.15 g of LiCl