Find the molar mass of calcium acetate...158.165g.
So start with 255g Calcium acetate x 1 mole Calcium Acetate/158.165 g Calcium Acetate x 6 mol Hydrogen/1 mole Calcium Acetate x 6.022e23 atoms/mole.
So...(255 x 1 x 6 x 6.022e23)/(158.165 x 1 x 1) = 5.825e24 hydrogen atoms